Purpose: This study examined the effect of selecting a single-peak blue vs a polywave blue/violet emission LED curing light on the degree of conversion (DC) and Knoop microhardness (KHN) of resin cements when light cured through a ceramic disk. Materials and Methods: Two shades (A1 and A4) of resin cement (Variolink II) were placed in a 0.5-mm-thick ring. The top surfaces were covered with a Mylar strip and further covered with a disk of 1-mm-thick Empress Esthetic ceramic, shade A2. The specimens were light cured by means of an Elipar-S10 (3M ESPE, single-peak blue LED) or BluePhase-G2 (Ivoclar Vivadent, polywave blue/violet LED) curing light, both for 20 s, directly on the surface of an attenuated total reflectance FT-IR plate at 30°C. The DC of the resin was calculated after 100 s. The specimens were removed, and the Knoop microhardness was tested immediately and again after 24-h storage in the dark at 37°C and 100% humidity. Five specimens were made in each group. The DC and Knoop microhardness results were analyzed with ANOVA and Fisher's PLSD at α = 0.05. Results: The choice of curing light had no significant effect on the DC and only a small effect on the immediate and 24-h KHN values. Shade A4 of the resin cement was harder and had a higher DC than shade A1. Conclusion: When light cured for 20 s, Variolink II resin cement can be light cured with either the single-peak or the polywave curing light. Shade A4 of the cement was slightly harder than A1. Keywords: polymerization, cement hardness, degree of conversion, LED, ceramic