Objective: The aim was to evaluate the influence of fluoride-releasing restorative materials in enamel and dentinmicrohardness. Material and Methods: 40 blocks (5x5x3 mm) from cervical third of human molars received a cavitypreparation between the enamel and dentin, and the restorations were subjected to in vitro caries model. Specimenswere randomly restored with (n=10): conventional glass ionomer cement (Ketac Cem, 3M ESPE); polyacid-modifiedcomposite resin (Ionoseal, VOCO); resin-modified glass ionomer cement (Ionofast, Biodinâmica); or microhybridcomposite resin (Filtek Z250, 3M ESPE). The specimens were sectioned longitudinally and enamel and dentinKnoop microhardness were determined at different distances from the restorative material (100, 200 and 300 um)and depth of surface (20, 40 and 60 um). The data were submitted to three-way repeated measures ANOVA andTukey´s test (a=0.05). Results: For enamel, the double interactions between material x distance and material xdepth were statistically significant. In all depths and distances, the highest values of enamel microhardness wereobserved for Ketac Cem. In dentin, the materials differed statistically from each other, and Ionoseal obtained highermicrohardness values than those found in Ionofast. Conclusion: Conventional glass ionomer cement is more effectivein preventing enamel demineralization around restoration followed by the polyacid-modified composite resin. Indentin, the polyacid-modified composite resin obtained better performance than resin-modified glass ionomer cement.KEYWORDSCompomers; Dental caries; Glass ionomer cements; Hardness tests; Composite Resins.