The aim of the study was to compare the monomer conversion and cytotoxicity of two resin composites, Tetric Ceram (TC) and Heliomolar (HM), irradiated with a halogen light-curing unit (Optilux 501) using a number of curing modes, boost (OB), ramp (OR), and conventional (OC), as well as a light-emitting diode (LED) curing unit (LEDemetron; LEDe). The effects of irradiation times (10, 20, 30, and 40 s) were also investigated. The monomer conversion of resin composites was measured using Fourier transform infrared (FTIR) analysis. Resin samples were preimmersed in Dulbeco's modified engles medium (DMEM) for 24 h, and the periodontal ligament cells (PDL) were cultured with aged medium for 72 h to evaluate the cytotoxic effects of resin samples using the microtiter-tetrazolium (MTT) method. The spectral outputs of the curing units were also compared. After irradiation using the same curing mode, the TC monomer conversion was higher than the HM analog. The TC conversion value was lower after OC and LEDemetron irradiation for 10 s compared to exposures of 30 and 40 s. The HM conversion value was lowest for the 10-s OC irradiation, and highest for the 40-s OC exposure. In cytotoxicity tests, the negative control group was without irradiation. The succinate dehydrogenase (SDH) activities of the OB and OR-irradiated TC samples were significantly lower than analogs irradiated with LEDemetron for 40 s; however, no significant differences were demonstrated between OB, OR, and control groups. Further, no significant differences in SDH activity were demonstrated for OB, OR, and control groups of the HM.