Objectives: This study was aimed to investigate the degree of monomer conversion of three dual-cure resin cements irradiated through various restorative materials or dentine.Methods: Dual-cure resin cements RelyX ARC (3M-ESPE), RelyX Unicem (3M-ESPE) and Variolink 2 (Ivoclar,Vivadent)  was used for this study. The cements were mixed according to the manufacturer' instructions and placed under the discs (1.5 mm thickness) representing a metal restoration, composite restoration (Sinfony D A3), fiber-reinforced composite (FRC) (EverStick 1.0 mm+ 0.5 mm Sinfony D A3) restoration and dentin. Five specimens (0.6mm thick) for each group were irradiated through the discs for 40s (Optilux-501, 800 mW/cm²).  Light polymerization of the dual-cure cement without the covering disc was used as control. Degree of monomer conversion (DC%) was determined by FT-IR/ATR spectrometry from the bottom of the cement. The infrared spectra were recorded at every 6 sec for fifteen minutes beginning after mixing the cements.Results: ANOVA revealed significant differences in DC% between the cements tested and the different restorations (p<0.05). The mean DC% (SD) were:

 Conclusion: Light irradiation through FRC-disk revealed highest DC% among groups and was comparable to control. It can be concluded that the simulating material of the restoration significantly influence the DC% of the dual-curing cement underneath.