Context:Achieving a high degree of conversion (DC) is one of the major concerns during photopolymerization of bulk-fill composites.Aims:To evaluate the effect of light energy densities (11.2 J/cm2 and 20 J/cm2) on the DC and variation of DC in the 24-h postcuring of four bulk-fill composites: SDR, Venus Bulk Fill, MI FIL, and Tetric N-Ceram Bulk Fill at simulated clinically relevant filling depths.Settings and Design:This was an in vitro comparative study.Subjects and Methods:A total of twenty samples were prepared using a teflon mold. VALO curing light was used with two light intensity modes of 1000 mW/cm2 for curing time of 20 s and 1400 mW/cm2 for curing time of 8 s. The energy density was calculated as follows: energy density (J/cm2) is the light intensity (mW/cm2) applied during a certain time (s) divided by 1000. The DC was measured at two time intervals: immediately postcure and after 24-h storage in artificial saliva using an Fourier-transform infrared spectroscopy equipped with attenuated total reflectance accessory.Statistical Analysis Used:ANOVA and Bonferroni test at P < 0.05.Results:High energy density (20 J/cm2) leads to higher DC. Thickness, type of composites, and postcuring phase strongly influence the DC. DC values of the top surface for all the bulk-fill materials investigated were found significantly greater (P < 0.005) than those of their bottom surface. Among composites, SDR showed highest DC. DC strongly increased after 24-h postcure by 32% on top surface and 76% on bottom surface.Conclusions:Energy density more than 20 J/cm2, derived by increasing curing time and low power density, helps obtain a high DC of bulk-fill composites for adequate clinical performance.