This study compared the effect of water sorption on the extent of marginal gap reduction in two resin-modified glass-ionomer cements (RMGICs), two giomers, two compomers, and two resin composites over a twelve-week storage period. Artificial gaps were created in 160 borosilicate glass cylinders. One-half of the internal surface of each cylinder was blocked out with wax and the other half was sandblasted. The bonding surface was further treated with 4% hydrofluoric acid, rinsed, and then coated with silane. After removal of the wax, one coat of dentin adhesive was applied to the silane-treated surface of the cylinder, briefly air dried and light cured. Eight light-cured restorative materials were placed incrementally: Vitremer (V), Fuji II LC (FJ), Beautifil (B), Reactmer Paste (R), Compoglass F (C), F2000 (F), Filtek Z250 (Z), and Tetric-Ceram (T). For each material, ten specimens were stored in deionized water (W), and ten (control) in nonaqueous silicone fluid (O) at 37°C. The dimension of the same maximum gap created in each specimen was repeatedly measured at 0, 1, 2, 4, 6, 8, 10, and 12 weeks. R-W exhibited extensive hygroscopic expansion that resulted in cracking of 40% of glass cylinders after the 2nd week and 70% after the 4th week. One-way ANOVA of the other seven water groups showed significant differences (p 0.001) among gap widths measured at different time intervals in V-W, FJ-W, C-W, F-W. Both RMGICs had the most significant gap reduction during the first week (p 0.001). Both compomers exhibited delayed water-sorption characteristics, with more significant gap reduction observed in C-W. B-W was similar to the two resin composites Z-W and T-W and exhibited the least gap reduction. After the first week, there were no significant differences in the percentage reduction in marginal gaps for any of the groups (p > 0.05). Marginal gap reduction that results from water sorption is more extensive and rapid in RMGICs, followed by compomers, whereas composites are relatively stable. Reactmer Paste exhibits rapid and extensive expansion and should probably be avoided in tooth preparations that involve thin unsupported enamel.