This study investigated the effect of 3 staining solutions and 3 over-the-counter tooth-bleaching systems on the microhardness of 2 dental resin composites. Forty-five specimens of Filtek Supreme and Esthet-X were randomly assigned to 3 groups. Over a 40-day test period, the specimens in each group (n=15) were immersed in 1 of the 2 staining solutions (coffee and red wine) or distilled water as the control for 3 hours a day at room temperature. The 15 specimens in each staining group were further randomly divided into 3 subgroups, and the specimens in each subgroup (n=5) were bleached using one of the bleaching agents (Night Effects, Simply White Night and Opalescence Quick). Surface hardness was measured at 24 hours after polymerization (baseline), after staining and after bleaching. Means and standard deviations were calculated, and the data were analyzed using repeated-measures analysis of variance and Duncan's Test. The microhardness of Esthet-X was significantly higher than Filtek Supreme at baseline (p<0.01). All specimens of both materials immersed in coffee and wine revealed a significant hardness decrease compared to baseline values (p<0.05). In the control group, microhardness was increased, and this increase was statistically significant for Filtek Supreme (p<0.05). After bleaching, there was a significant decrease in mean microhardness for all groups tested (p<0.05). No significant difference was found among bleaching agents.