We performed X-ray photoelectron spectroscopy (XPS) to gain a more in-depth understanding of the chemical composition of COF-SO3H. The main component of the high-resolution N 1s spectra (Figure ), observed at a binding energy of 400. and groups -CHO of DASA and Tp, respectively. The high-resolution S 2p spectrum revealed a typical asymmetric peak for a sulfonic acid moiety (Figure ), and the peaks at binding energies of 168.4 eV and 169.4 eV are assigned to S 2p3/2 and S 2p1/2, respectively. The surface and bulk chemical composition determined by XPS and elemental analysis, respectively is provided in Table , accompanied by a brief discussion of the result. Powder X-ray diffraction (PXRD) (Figure ) revealed the crystalline nature of pristine COF-SO3H and agrees well with previous reports as well as the simulated spectrum for an eclipsed 2D stacking model structure. Diffraction peaks present at 2θ = 4.6°, 8.1° and 26.7°, were attributed to reflection from the (100), ( ) and (001) planes, respectively. The broadening of the PXRD peaks can be attributed to a combination of small particle size and small crystalline domains. Thermogravimetric analysis in air confirmed high stability of COF-SO3H, up to a temperature of ~200 °C (Figure ). The network exhibits a three-stage weight loss, corresponding to the removal of adsorbed water (50-120 °C), decomposition of sulfonic acid groups (200-370 °C) and the disintegration of the remaining framework (>370 °C). N2 gas physisorption analysis performed at -196 °C yielded a type I isotherm (Figure ), typical for microporous solids. We assume that the gap between adsorption and desorption originates from the swelling behavior of COF-SO3H. The pronounced porosity manifests in a Brunauer-Emmett-Teller specific surface area (SBET) of 280 m 2 g -1 and a total pore volume of We exposed dry COF-SO3H to 10, 20, 30, and 90% RH to investigate the water sorption/desorption behaviour in more detail (Figure ). Upon exposure to 10% RH, COF-SO3H adsorbed ~0.13 g•g -1 after 24 hours. Notably, 80% of the water adsorption took place within 30 min, and 95% occurred after one hour, demonstrating fast uptake at low RH. Water desorption was triggered by a reversal to 0% RH, and 0.12 g•g -1 or 93% of the adsorbed water was removed within 2 h. Comparable adsorption rates were observed at 20%, 30%, and 90% RH, reaching total capacities of 0.17 g•g -1 , 0.19 g•g -1 and 0.32 g•g -1 , respectively.