The goal of this work is to determine the properties of the stellar populations in a sample of LIRGs and ULIRGs. Using the ages as a clock we investigate: a) whether LIRGs-ULIRGs evolve into Radio Galaxies and QSOs; b) whether cool LIRGs-ULIRGs can evolve into warm LIRGs-ULIRGs; c) the merger sequence deduced from the morphological studies is reflected in the properties of the stellar populations. Using evolutionary synthesis models with high spectral resolution stellar libraries we have found that the intermediate age stellar population dominates at optical wavelengths. The stellar population in LIRGs is similar to ULIRGs and ULIRGs-QSOs transition objects.