Objectives: This in vitro study assessed the effect of different light-curing units on the elution of monomers from bulk fill composites with different thicknesses. Methods: Five bulk fill composites (Filtek Bulk Fill Flowable, SonicFill 2, SDR, Tetric N-Ceram Bulk Fill, and Venus Bulk Fill) and one conventional composite (Filtek Z250) were selected for the study. The cylindrical specimens in thickness of 2 mm or 4 mm were prepared and photopolymerized for 20 s with a light-emitting diode (VALO Cordless) or a halogen (Monitex BlueLuxer) light-curing unit. The specimens in glass vials were covered with a 75% ethanol/water solution. Ethanol/water extraction solutions were collected for high-performance liquid chromatography analysis after 24 h, 3 days, and 7 days. The data were analyzed with repeated measures and three-way ANOVA (p < 0.05). Results: The total monomer amount was significantly influenced by light-curing source used and thickness. The highest levels of Bis-GMA and Bis-EMA were eluted from Tetric N-Ceram BulkFill and Venus Bulk Fill, respectively. SonicFill 2 released the highest level of TEGDMA at 4-mm thickness. The highest levels of UDMA release, from 4-mm-thick Filtek Bulk Fill Flowable, were attained using the halogen unit. Conclusions: Light-curing sources affected the number of monomers released by materials. The amount of eluted monomers declined over time. The increased ratio of released monomers to increased thickness is material dependent. The number of residual monomers is highly associated with the resin ratio and crosslinking network of the composites.