The value of the permeability coefficient 𝐴 for the mixed methanol-water solvent was determined as 4.5×10 -7 m 3 m -2 s -1 bar -1 . This is only slightly lower than the system's pure water permeability, estimated as 4.7×10 -7 m 3 m -2 s -1 bar -1 from the stabilised pure water flux across BW30 during membrane preconditioning at 30 bar (SI Section S2), which is reasonable given the presence of methanol in the mixed-solvent. The value of the solute permeability coefficient 𝐵 was regressed as 1.5×10 -8 m 3 m -2 s -1 . Using the obtained values for 𝐴 and 𝐵, the selectivity for this system was calculated as 𝑆 = _ R ≈29.9 bar -1 . This order of magnitude is in line with commonly observed selectivity values for polyamide RO membranes. The obtained reflection coefficient of 𝜎 = 0.99 indicates near-perfect permselectivity of the membrane. A small deviation from ideal 𝜎 = 1 may be explained by the presence of inherent