Hydrogel membranes formed by interfacially photopolymerizing poly(ethylene glycol) (PEG) diacrylate precursor solution were prepared from PEG diacrylate of molecular weights (MW) ranging from 2000 (2K) to 20000 (20K) with concentrations ranging from 10% to 30% w/w. The effects of PEG diacrylate MW and concentration in the membrane precursor solution upon the diffusivities of vitamin B12, myoglobin, ovalbumin, albumin, and IgG were determined. Regardless of the concentration of the PEG diacrylate in the precursor solution, hydrogels prepared with PEG 2K, 4K, and 8K diacrylate were impermeable to proteins with a size equal to or larger than myoglobin (22 kDa), while hydrogels prepared with PEG 20K diacrylate were impermeable to proteins with a size equal to or larger than ovalbumin (45 kDa). Similarities between hydrogels formed from PEG 2K, 4K, and 8K diacrylates were also seen in calculations of the molecular weight between crosslinks and the mesh size, with values in the range of 150-750 g/mol and 15-35 A, respectively, depending on PEG diacrylate concentration. In contrast, hydrogels formed from PEG 20K diacrylate had molecular weight between crosslinks ranging from 1150 to 2000 g/mol and mesh sizes ranging from 45-70 A, with larger values being observed in membranes polymerized from more dilute PEG diacrylate precursor. [on SciFinder (R)]