Films were prepared by reaction of HEMA monomer with the crosslinking agent ethyleneglycol dimethacrylate (EGDMA) at crosslinking ratios, X, of 0.005, 0.01, 0.0128, 0.025, and 0.050 mol EGDMA/mol HEMA in the presence of 40 wt.% water at 60 degrees C for 12 h. These membranes were subsequently swollen in water at 37 degrees C and their structure analyzed using a modified Gaussian distribution equation of equilibrium swelling. The calculated values of Mc varied between 800 and 3700 daltons, which corresponded to a correlation length of the mesh size xi of 16.2 to 35.6 A. The structural analysis and diffusive studies of PHEMA membranes indicate that the recent determinations of Mc for PHEMA by Migliaresi et al. (C. Migliaresi, L. Nicodemo, L. Nicolais, and P. Passerini, "Physical characterization of PHEMA gels," J. Biomed. Mater. Res., 15, 307 (1981). and others are not accurate. The methods presented can also be used for analysis of any highly crosslinked polymer network produced by simultaneous polymerization and crosslinking reactions.