INTRODUCTION: Bleaching may exert some negative effects on existing resin restorations. OBJECTIVES: To evaluate the effect of bleaching with 14% and 40% hydrogen peroxide on microleakage and microhardness of different tooth-colored restorations. MATERIALS AND METHODS: MICROLEAKAGE TEST: Class V cavities were prepared on labial surfaces of 60 extracted human upper central incisor teeth. The teeth were divided into 2 groups: Group I: restored with FiltekZ350XT composite, Group II: restored with Fuji II LC resin-modified glass ionomer. The teeth were thermocycled, each group was subdivided into 3 subgroups: subgroup A: was not bleached and served as control, subgroup B: bleached with 14% hydrogen peroxide gel and subgroup C: bleached with 40% hydrogen peroxide gel. The teeth were immersed in dye, sectioned, and dye penetration was scored at the incisal and cervical walls under stereomicroscope. Data were analyzed using Repeated Measures ANOVA, Wilcoxon and Mann-Whitney tests. MICROHARDNESS TEST: 20 specimens (2mm thickness and 10mm diameter) were prepared from Filtek Z350XT composite, and Fuji II LC RMGI. Specimens were subjected to thermocycling, after which the microhardness of each specimen was measured before bleaching. Specimens were subdivided in to 2 subgroups, and bleached with 14%, 40% hydrogen peroxide gels. After bleaching, microhardness of each specimen was measured again. Data were analyzed using independent sample t-test and paired t-test. RESULTS: For microleakage test; statistical analysis showed no significant differences in microleakage of the tested composite and RMGI subgroups for incisal or cervical margins. For microhardness; the results showed significant increase in mean microhardness for the composite and RMGI subgroups bleached with 14% hydrogen peroxide, whereas, composite and RMGI subgroups bleached with 40% hydrogen peroxide showed significant decrease in mean microhardness. CONCLUSION: Bleaching did not have an effect on microleakage of Filtek Z350XT composite and Fuji II LC RMGI restorations, while they affected the microhardness of these restorations.