Objective: Two recently marketed light-emitting diode polymerization units were compared with a conventional halogen curing unit. Selected properties of resin composites polymerized with the three curing units were determined. Method and materials: Three different brands of resin composite were used in the investigation. The properties that were determined were flexural strength and modulus, determined in three-point bending tests; depth of polymerization, assessed by removal of uncured material after irradiation; polymerization contraction, evaluated with the bonded-disk method; and degree of conversion, measured by Fourier transform infrared spectroscopy. Results: The properties of resin composites polymerized with light-emitting diode curing units were equal or inferior to properties obtained after halogen light curing. Conclusion: Although several properties with some combinations of resin composite and light-emitting diode curing unit were inferior to properties obtained with the halogen curing unit, both the flexural strength and the depth of polymerization fulfilled the requirements set by the International Standards Organization.