Objectives: The aim of this study was to evaluate, in situ, the microhardness on dental enamel submitted to different treatments: Methods: 1 - in-office bleaching with 37% carbamide peroxide-CP37 (Whiteness Super-FGM); 2 - at-home bleaching with 10% carbamide peroxide-CP10 (Whiteness Perfect-FGM); 3 - the association of both; and 4 - a placebo agent-PLA (Carbopol-FGM) as control. Enamel fragments measuring 3x3mm were submitted to sequential polishing. Initial (I) microhardness test were performed with a Knoop diamond in a microhardness tester with 25g load. The fragments were fixed on the vestibular surface of the first superior molars of 44 volunteers. The volunteers were divided in four groups according to the treatment (n=11): G1-CP37 and CP10; G2-CP37 and PLA; G3-PLA and CP10; G4-PLA and PLA. The in-office bleaching was performed weekly with 2 applications of the bleaching agent in the same session for 1h and the at-home bleaching was done by volunteers in nighttime for around 8 hours daily. After 3 weeks, the fragments were removed and final (F) microhardness were obtained. Results: ANOVA and Tukey's test (a=0.05) showed no differences among initial or final microhardness values, however significant differences occurred between initial and final intra-groups values: G1(I)= 390.0 A; G1(F)= 363.3 B; G2(I)= 396.6 A; G2(F)= 380.3 B; G3(I)= 395.5 A; G3(F)= 381.3 B; G4(I)= 396.1 A; G4(F)= 382.2 B (LSD= 9.3). Conclusions: The results indicated that the in-office, at-home, association of both bleaching techniques and, also the treatment with placebo agent (Carbopol) decreased the enamel microhardness.

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