Two polyacid-modified composite resins, Dyract and Compoglass, have been studied for water-uptake on storage in three aqueous media, namely pure water, 0.9% NaCl and 1 M NaCl at 37 degrees C, and the results compared with those of a conventional composite resin, Pekafill. The equilibrium water-uptake of Dyract and Compoglass varied depending on time of cure and ionic strength of the storage medium. This latter finding contrasted with that for Pekafill, where equilibrium water content showed almost no variation with ionic strength of the storage medium. Uptake of water was always greater in Dyract and Compoglass than in Pekafill, reflecting the role of water in promoting the later neutralization reaction in these materials and its further role in hydrating the ionic products of that reaction. Despite this and other evidence of neutralization, specimens of Dyract and Compoglass stored in wet conditions (0.9% NaCl) showed no change in compressive strength on storage from 24 h to 30 days, a result similar to that reported previously for Dyract alone for flexural strength for times up to 1 year. By contrast, specimens of Dyract and Compoglass stored in dry air at 37 degrees C, and therefore unable to undergo any neutralization, showed an increase in compressive strength of between 40 and 70 MPa by 30 days that was significant at P < 0.01.