AbstractPolymethacrylate networks were made by copolymerization of a range of compositions of bis‐GMA and triethylene glycoldimethacrylate (TEGDM). Polymerization was initiated both by heating with benzoyl peroxide or by photopolymerization (λ > 400 nm) using camphoroquinone as sensitizer. The uptake of water increased from 3 to 6% as the proportion of TEGDM increased from 0 to 1.0. Intermediate compositions took up less water than would be predicted from the law of mixtures. Volumetric changes were determined and clinical significance discussed. A copolymer prepared by photopolymerization took up more water as the temperature was increased from 24–60°C. In this range, values of the diffusion coefficient (D) conformed to the Arrhenius equation, D = D0 exp (−E/RT), giving E = 42–46 kJ/mol and D0 = 0.13 cm2 s−1.