Objective: The goal of this investigation was to assess in vitro the quality of marginal sealing of composite-dentin adhesive systems and human dentin. Method and materials: Forty intact human premolars and third molars were extracted from subjects of both sexes and of different ages. After the enamel layer was removed, a Class V cavity was formed on the buccal surface, and a wedge cavity was formed on the lingual surface. These were restored with resin composite materials and their corresponding dentin adhesive systems. The quality of marginal sealing was evaluated by assessing the linear penetration of silver nitrate dye. Results: The best marginal sealing between composite materials and the cavity walls, in both wedge erosions and Class V cavities, was provided by Scotchbond Multi-Purpose/Valux and Syntac/Helioprogres systems. Dye penetration was slightly greater with the XR-Bond/Herculite, Gluma/Pekafill, and Superlux Universal Bond 2/Superlux Solar systems. The greatest microleakage was observed in Tripton/Opalux and Denthesive/Charisma specimens. Conclusion: The use of an adhesive system and the corresponding resin composite does not eliminate microleakage completely when the cavity margins are in dentin.