Abstract The effect of the morphology on the hygroelastic behaviour of polyester and epoxy resins is studied. The morphology is expressed by two factors, namely, the free volume of the polymer and a two-phase formation. The first factor is varied in styrene-crosslinked polyester using different styrene/alkyd proportions, and by using different immersion temperatures. The second is investigated in polyesters by replacing styrene with bromostyrene inducing a two-phase structure, and in epoxies by changing the hardener/epoxy ratio. It is shown that the hygroelastic response is affected markedly by the morphology. In general, both the moisture content and its rate of absorption are higher when the polymer network is more open and when more free volume is available. In some cases, however, the hygroelastic parameters are dominated by a two-phase structure where present.