A series of cross-linked polymers, XLPEGDA, was prepared by photopolymerizing poly(ethylene glycol) diacrylate (PEGDA) in the presence of varying amounts of water or monofunctional poly(ethylene glycol) methyl ether acrylate (PEGMEA) to vary cross-link density. All of the polymers had essentially the same chemical composition but displayed a systematic variation in cross-link density as estimated from water swelling experiments, models such as the Flory-Rehner and modified Flory-Rehner theories, and dynamic mechanical testing. Cross-link density decreased with increasing water or PEGMEA content in the prepolymer solutions. Interestingly, gas solubility, diffusivity, and permeability were essentially independent of cross-link density for the series of materials prepared from PEGDA and water. The polymer density, fractional free volume, glass transition temperature, and polymer d spacing were also constant when water was used to vary cross-link density. On the basis of this result, it appears that cross-link density does not necessarily affect gas diffusion and permeation, the polymer glass transition temperature, or the fractional free volume in network polymers, and ascribing changes in these properties to changes in cross-link density alone, as is seen commonly in the literature, should be done with great care.