Purpose: The objective of this study was to determine radiopacity values of a group of resin-based cements and to compare them to those of enamel and dentin. Materials and Methods: Three specimens 2.5 mm thick were prepared from each of 18 cements (nine chemical-cured and nine dual-cured). Three tooth sections 2.5 mm thick were cut from extracted molars. Specimens and tooth sections were divided into three matching groups. Following standard radiographic techniques, images of each group of specimens, along with an aluminum step wedge were obtained. Optical density readings for each material were determined with a transmission densitometer. Radiopacity values were subsequently calculated as equivalents of aluminum thickness. Results: Analysis of variance indicated significant differences in radiopacity values among the materials (P 0.0001). Four dual-cured cements (Variolink, Geristore, Enforce, and Nexus) had radiopacity values significantly greater than that of enamel, while two (Choice and Adherence) had radiopacity values similar to enamel. For the remaining three dual-cured cements, Duolink had a radiopacity value significantly lower than that of enamel but higher than that of dentin, while both Lute-It and Resinom er had values similar to dentin. For the chemical-cured cements, five materials had radiopacity values significantly higher than enamel (Sealbond, Advance, Scotchbond, Cement-It, and Dyract-Cem), while two had values significantly lower than enamel but higher than dentin (Biomer and Panavia 21). The remaining two cements had radiopacity values significantly lower than dentin. Conclusion: For a group of 18 resin-based cements, some had radiopacity values similar to or significantly lower than that of dentin.