Objective: To compare the polymerization effectiveness of two resin composites cured with a quartz tungsten halogen (QTH) lamp or a light emitting diodes (LED) unit. Study design: Filtek Z250 (3M ESPE) and Spectrum TPH (Dentsply DeTrey) resin composites were placed in 9 mm deep and 4 mm wide metallic molds and cured using the QTH light Hilux 200 (Benlioglu) or the LED unit Smartlite IQ (Dentsply DeTrey) for 20 or 40 s (three specimens per group). Measurement of depth of cure was carried out by means of a scraping technique, according to ISO 4049. The microhardness measurements were performed using a calibrated Vickers indenter (100 g load, 30 s) at depths of 0.5, 1.5, 2.5, 3.5, 4.5 and 5.5 mm from the top of the composite in the same specimens. Results were analyzed by ANOVA, Student ́s t and Student-Newman-Keuls tests (p<0.05). Results: Filtek Z250 exhibited higher depth of cure and Vickers microhardness values than Spectrum TPH under each experimental condition evaluated. Depth of cure and microhardness were not affected by the curing light used. However, hardness values were influenced by the interaction between curing light and exposure time. Specimens irradiated for 20 s exhibited higher microhardness values when the LED curing light was used. Exposure time had no influence on the microhardness values for depths from 0.5 to 2.5 mm. At higher depths, irradiation for 40 s produced greater microhardness values. Conclusions: Curing effectiveness of resin composite is not only dependent on the curing light unit. Results vary greatly with composite brand, thickness of the resin composite and the duration of the exposure.