In this paper the alteration of asphalt cements that occur during the manufacture of paving mixtures in the plant mixing operation was studied, by laboratory testing. The bituminous mixtures were prepared in a laboratory mixer under controlled conditions (time and temperature). The asphalts under test were recovered by means of the Abson method. The alteration was studied measuring the viscosity with the sliding plate microviscometer and by infrarred spectra. Furthermore a study of the action of inhibitors of oxidation was made. This test was conducted to determine the effectiveness and the mechanism by which the antioxidants perform.