In recent years, covalent organic frameworks (COFs), as a class of crystalline porous organic materials, have emerged as a new type of membrane materials with great potentials due to their precisely pre-designable pore structures and tailored functions. To overcome the limitation that COFs are usually synthesized as insoluble and unprocessable powder, interfacial polymerization has been implemented for the fabrication of COF membranes. In a typical process, monomers are dissolved separately in two immiscible phases, and a polymerization reaction takes place at their interface to produce a COF membrane over time. This strategy has achieved considerable success in construction of COF membranes. However, it is hard to fabricate Janus membranes because both the two sides of the as-formed membranes have the same composites. Although formation of Janus membranes via this approach was serendipitously observed, this phenomenon was not universal and so far only a few cases was reported, for which the difference in the membrane surfaces was attributed to different diffusion rates of the monomers. Since such a diffusion-controlled structural alteration is susceptible to environmental influences, it is non-designable and uncontrollable. To address this problem, a designable and controllable strategy for constructing asymmetric COF membranes should be based on chemical composition control, wherein components with different wettability could be introduced on-demand to the upper and lower surfaces of a membrane, respectively. In this study, we have introduced an innovative method termed "multiphase interfacial polymerization" to fabricate Janus COF membranes with asymmetrical wettability. As depicted in Scheme 1, dialdehydes modified with hydrophobic alkyl side chains (Cl-TPA) and hydrophilic oligo (ethylene glycol) side chains (O-TPA) were dissolved in dichloromethane and toluene, respectively, which served as two oil phases. Additionally, a co-precursor, triamine (TAPB), was dissolved in water to form an aqueous phase which locates between the two oil phases. By adjusting the quantity of water and the composition of the additives in the aqueous phase (see Supporting Information (SI) for details), we successfully fabricated a Janus COF membrane (denoted as LY-Janus) with asymmetrical wettability from a toluene-water-dichloromethane three-phase system. As observed, at the early stage of the multiphase interfacial polymerization process, two thin films initially formed at the toluene/water and water/dichloromethane interfaces via independent interfacial polymerizations between the amine monomer and the corresponding aldehyde monomers, respectively. Subsequently, the two polymerization processes continued as the two aldehyde monomers diffuse crossing from the films via opposite direction. As a result, the two films grew thicker and thicker, and they became closer and closer to each other, with gradual outdiffusion of water. Finally, the two films merged each other to produce a monolithic three-component Janus membrane. For comparative purpose, two twocomponent COFs membranes, one with hydrophobic alkyl side chains (LY-SH) and the other with hydrophilic oligo (ethylene glycol) side chains (LY-QH) were also prepared using the conventional two-phase interfacial polymerization method (see SI for details). The as-obtained membranes were characterized with FT-IR and solid-state C NMR spectroscopy, which confirmed the formation of imine polymers. The comparison of FT-IR spectra of the membranes and their corresponding monomers revealed disappearance of the vibration peaks corresponding to N-H (3341 cm -1 ) and C=O (1680 cm -1 ) and appearance of C=N vibration band (1617 cm -1 ) after the condensation reactions (Figures S12-14), indicating the formation of polyimine structures. Moreover, the FT-IR spectrum of LY-Janus looks like a superposition of the spectra of LY-SH and LY-QH (Figure ), suggesting that it contains the constituents from both the two membranes. The formation of polyimines was also supported by solid-state C CP/MAS NMR spectroscopy, which displays the characteristic signal of C=N at 153 ppm in their C CP/MAS NMR spectra (Figure ). Particularly, the comparison of their C CP/MAS NMR spectra clearly indicates that the Janus COF membrane consists of the chemical components from both the monomers Cl-TPA and O-TPA, as evidenced by the co-appearance of the peaks corresponding to the alkyl chain (69, 45, 33, 30, and 28 ppm) and the oligo (ethylene glycol) segment (71 and 59 ppm) (Figures ). Powder X-ray diffraction (PXRD) corroborated the crystallinity of the three membranes. And their crystal structures were elucidated on the basis of the experimental PXRD data and theoretical simulations (Figures S18-23). A close comparison of the (100) diffraction peaks of the three membranes provides more information (Figure ). Firstly, LY-SH and LY-QH display a slight but clear difference in the position of their (100) peaks, albeit they share the same framework skeleton. It could be attributed to their different side chains, for which the influence of substituents on the PXRD diffraction was observed in previous studies. Secondly, the (100) peak of LY-Janus lies between the peaks of LY-SH and LY-QH, which is consistent with the fact that it contains both the frameworks of LY-SH and LY-QH, again corroborating the formation of the designed three-component COF membrane. More characterization was carried out to disclose the microstructures and features of the COF membranes. Thermal gravimetric analysis (TGA) revealed that the membranes had good thermal stability (Figure ). Nitrogen sorption measurements confirmed their porosity, with BET surface areas of 89, 135, and 244 m 2 •g -1 for LY-SH, LY-QH, and LY-Janus, respectively (Figures ). The low surface areas could be attributed to occupation of their pores by the long side chains. Moreover, their pore size distribution profiles indicate that they possess narrow distribution and very close pore sizes (around 24 Å) (Figures S28-30), indicating well-ordered channels in the membranes. The surface morphology of the three membranes was investigated with scanning electron microscopy (SEM). As shown in the SEM images, the upper surface of LY-Janus displays a morphology of aggregated nanosheets (Figure ), which closely resembles to the surface of LY-QH (Figure ). This surface exhibits a huge roughness and dense texture, facilitating spreading of water droplets on the membrane surface. In contrast, the lower surface of LY-Janus shows a morphology of aggregated nanospheres (Figure ), which is similar to the lower surface of the LY-SH membrane (Figure ). This result is in accordance with the