Purpose: To evaluate the effect of different polymerization techniques on microleakage and gap formation of resin-based composite restorations. One correlation test was also carried out between these methodologies. Methods: 180 vertical slot cavities were prepared in bovine teeth and filled with Z250/Single Bond system, for the following six groups (n=30): Soft-start I - 10 seconds at 75 mW/cm(2) + 30 seconds at 560 mW/cm2; Soft-start II - 10 seconds at 190 mW/cm(2) + 30 seconds at 560 mW/cm(2); Pulse delay - 3 seconds at 300 mW/cm(2) + 5 minutes waiting + 30 seconds at 560 mW/cm(2); Plasma arc - 3 seconds at 1,500 mW/cm(2); High intensity - 40 seconds at 810 mW/cm(2); Conventional (control) - 40 seconds at 560 mW/cm(2). After polishing, epoxy replicas were prepared for electron microscopy analysis (n=10), at x500 magnification. The samples were thermocycled, new epoxy replicas were prepared, and the teeth were immersed in 2% methylene blue dye solution for 4 hours. Marginal gaps were evaluated again and the microleakage was assessed. Results: No statistical difference among polymerization techniques was observed for microleakage (Kruskal-Wallis test). For gap formation there was no statistical difference among polymerization techniques either before or after thermocycling (Tukey test). Conversely, thermocycling significantly increased the gap formation for all groups. No correlation (Spearman correlation test) was observed for the results of microleakage and gap formation.173156160