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Objectives: The aim of this study was to assess the effect of 30 % hydrogen peroxide on the microleakage of class V cavities restored with either a silorane-based compo-site or two methacrylate-based composites. Materials and Methods: A total of 96 standard class V cavities (1.5 × 2 × 3 mm) were prepared on the buccal surface of sound extracted human premolars with both enamel and dentin margins and randomly assigned into three groups of Filtek P90 (group A) with its respective bonding (P90 system adhesive), Filtek Z250 (group B) and Filtek Z350XT (group C), both with Adper Prompt L-Pop bonding. The teeth were subjected to thermocycling (1000×, 5–55ºC) and half of them randomly under-went bleaching (30 % hydrogen peroxide, 15 min, three times), while the remaining half (control) were not bleached. Dye penetration was measured following immersion in 2 % basic fuchsin for 24 h. Data were statistically analyzed using Kruskal-Wallis and Mann-Whitney U tests at 95 % CI. Results: No signi...