The purpose of this in vitro study was to determine the bonding qualities of three commercially available dentin bonding agents, namely Dentin Adhesit/Heliomolar, Creation Bond/Visar Fil and Scotchbond/P30. Microleakage tests and a scanning electron microscopy study of the interface between dentin and resin were performed. In eighteen specimens, the penetration of 0.5 percent basic fuchsin solution was studied across the enamel and dentin interfaces with the resin. Half of the specimens in each group of products were thermocycled. Leakage was evaluated after sectioning under a light optical stereomicroscope at 20 x magnification, using a classification reported by Eriksen and Buonocore. The interface between the restorative resin and the dentin axial walls of the preparation and its floor were examined by scanning electron microscopy (SEM). The results showed that leakage was commonly found at the resin/dentin interface and SEM showed gaps between the restoration and walls and floor of the preparation. It was concluded that the dentin bonding agents failed to provide a bond between restorative resin and dental tissue.