The aims of the present study were to evaluate the effects of various dentin chemical pretreatments (a) on the shear bond strength of five glass-ionomer cements (GICs) and (b) on marginal microleakage of the five GICs used in association with resin composites in Class V restorations. The dentin treatments were: three acid agents (polyacrylic acid, tannic acid, orthophosphoric acid), three cleansing agents (Tubulicid blue solution, hydrogen peroxide, and sodium hypochlorite), and an aqueous solution as control. After dentin treatment, the test specimens were stored in water at 37 degrees C for 24 hr. Shear bond strength was determined with a universal testing machine at a cross-head speed of 0.5 cm/min. Sodium hypochlorite and polyacrylic acid significantly improved the adhesion of GICs to a different degree in the various materials. Regarding microleakage tests, 320 non-retentive cavities were prepared at the cementum-enamel junction in freshly extracted human teeth. The teeth were thermocycled, immersed in dye solution, and serial-sectioned longitudinally at three sites. Treatment with sodium hypochlorite was the most effective in reducing marginal leakage. The present results suggest that dentin treatment is an important step in all resin composite/GIC restorations.