Objective: The purpose of this study was to evaluate the marginal microleakage and the extent of polymerization in Class II resin composite restorations prepared with two restorative techniques and two polymerization systems. Method and materials: One hundred twenty Class II cavities were prepared in bovine teeth and randomly divided into four groups: Bulk placement and conventional polymerization (Conv 1); buccolingual increments and conventional polymerization (Conv 3); bulk placement and soft-start polymerization (Soft 1); buccolingual increments and soft-start polymerization (Soft 3). All cavities were restored with the Z100/Single Bond system. After thermocycling, the specimens were immersed in 2% methylene blue solution and then evaluated for microleakage. Half of the samples were embedded in polyester resin and polished. The Knoop microhardness of the restorations was measured. Results: There was no dye penetration in 54.44% of Conv 1, 70.11% of Conv 3, 42.53% of Soft 1, and 63.22% of Soft 3 specimens. There were statistically significant differences in microleakage among groups. There were no statistically significant differences in microhardness among any groups at any depth. Conclusion: The incremental placement technique resulted in less microleakage. The soft-start system provided adequate polymerization but could not improve marginal sealing.