Dimethacrylate monomers polymerize to form highly crosslinked networks with an inhomogeneous distribution of regions of low and high crosslink density. The systems are probed in this work by observing the effects of solvents (nitrobenzene and water) on dynamic mechanical measurements, diffusion kinetics and fracture toughness experiments. Dynamic mechanical analysis showed that the addition of nitrobenzene is found to have little effect on the position of the glass transition or secondary relaxations for solvent levels of under 3.5 mass %. It is proposed that solvents initially diffuse into low crosslink density regions, impinging little on the methacrylate mainchains which reside largely in regions of high crosslink density. Only at higher solvent concentrations do these chains become plasticized. Confirmation of heterogeneity is provided by diffusion analysis of isothermal solvent uptake. This is also interpreted interpreted in terms of a two‐stage diffusion process due to differential rates of sorption which occur in different microregions. Weighing of the water uptake of samples suspended in water indicated that some 30% of water absorbed at equilibrium exists within void regions. Addition of water causes the fracture mechanism to change from a ‘stick‐slip’ mechanism characteristic of brittle polymers to stable crack growth with a lower fracture toughness. This is also attributed to differential swelling of the material causing internal stresses which aid material failure.