The post-gel strains of two light-polymerized resin composites having similar chemical composition, Filtek P60 and Filtek Z250, was assessed by strain-gauge analysis at the ex vivo level. The restoratives were tested in cavity factor 5 during light-polymerization and water-soaking for 24 h. Strain-gauge signals were digitalized by a data acquisition system and were displayed in a computer by corresponding software at a sample rate of 20 Hz. The strain data were used to compare post-gel strains, polymerization velocity, and total volumetric change of both materials. Evaluation of microleakage and scanning electron microscopy was also undertaken to elucidate effects of post-gel strains at the tooth-restorative interface. Microstrains of Filtek Z250 were lower than those of Filtek P60 and the differences between post-gel strains at several different time intervals were significant (P < 0.009). There was a correlation between polymerization velocity and total volumetric change. Microleakage evaluation and scanning electron microscopy did not reveal any signs of debonding at the interface for both materials. We conclude that the similarity in chemical composition of light-polymerized resin composites is not a determinant for post-gel strains. The tooth-restorative interface can withstand high post-gel strains arising from polymerization of resin composites.