Purpose: To evaluate the release of monomers from four different composite materials (Ceram X, Filtek Supreme XT, Tetric Flow, Tetric EvoCeram), polymerized using either halogen or LED unit. Methods: Ten specimens were made for each material/unit combination. Each specimen was stored in 1 ml 75% ethanol. The storage medium was renewed after 1, 7 and 28 days. Aliquots of this medium were analyzed by LC-MS/MS. Results: The effect of the curing unit on monomers' release differed significantly among the materials (P< 0.0001). The amount of BisGMA and TEGDMA released from Ceram X was not influenced by the unit used (P> 0.05). Curing with LED reduced the amount of Bisphenol A released from Ceram X compared to halogen. For Filtek Supreme XT, the type of unit exerted a significant effect on the elution of BisGMA (P< 0.05). LED curing resulted in a higher release of TEGDMA and UDMA compared to halogen (P< 0.05). For Tetric Flow, LED curing resulted in lower monomer release (P< 0.0001). For Tetric EvoCeram, the amounts of BisGMA, UDMA and Bisphenol A were higher when polymerizing with LED compared to halogen. The release of substances was more material dependent and less influenced by the curing unit used.