Objective: The purpose of this study was to evaluate the degree of conversion (DC) using FTRaman spectroscopy and the Knoop hardness (KHN) of composites cured by second and third-generation LED light curing-units (LCU), Radii Cal and Ultralume 5. Methods: Three composites (Filtek Supreme XT, Filtek Z350, and Esthet X) were selected for this study. KHN testing (n=10) was performed with 10 indentations for the top (T) and bottom (B) surfaces. For DC (n=10), both the T and B surfaces were analyzed. Results: For KHN, the three composites differed in hardens. There was a "LCU-surface" interaction, in whichRadii Cal showed significantly greater hardens in the B surface. For DC, there was a "composite-surface-LCU" interaction. For the "composite" factor, there was no significant difference between the groups, except for Supreme XT-Radii Cal (T or B surfaces). For the "LCU" factor there was a significant difference for Supreme XT T surface, Ultralume 5 obtained a higher DC. For the Z350 T surface, a significant difference in the DC in which Radii Cal obtained better results. For the "surface" factor, all groups presented T surfaces with a higher DC than the B surfaces, the sole exceptions involved Esthet X-Radii Cal and Z350-Ultralume 5. Conclusion: Knowledge regarding composite composition and the characteristics of LCUs are important for effective polymerization.64396401Jimenez-planas, A., Martin, J., Abalos, C., Llamas, R., Developments in polymerization lamps (2008) Quintessence Int., 39, pp. e74-e84Polydorou, O., Manolakis, A., Hellwig, E., Hahn, P., Evaluation of the curing depth of two translucent composite materials using a halogen and two LED curing units (2008) Clin. Oral. Investig., 12, pp. 45-51Cavalcante, L.M., Valentino, T.A., Carlini Jr., B., Silikas, N., Pimenta, L.A., Influence of different exposure time required to stabilize hardness values of composite resin restorations (2009) J. Contemp. Dent. 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