The polymerization shrinkage of light-cured dental composite resins has been reported to cause a marginal gap between the cavity wall and the restoration, leading to the premature and/or tensile stress failure of the composite restoration. This study measured the volumetric shrinkage of six light-cured posterior composites by measuring specific gravity differences between uncured and cured composite test specimens, using a modified version of ASTM method D792 "Specific Gravity and Density of Plastics by Displacement." The measured volumetric shrinkage ranged from 1.35% to 3.22%.