The effects of temperature and annealing on mechanical properties of (PbO) x (ZnO) (0.62x) (P 2 O 5) 0.4 glasses have been investigated using Brillouin scattering over the compositional range from x ¼ 0 to 0.6 between room temperature up to the softening point. The temperature dependence of the Brillouin frequency shift has permitted to determine the glass transition temperature and the softening point. The variations in these reference points and in the elastic constant C 11 with lead content are correlated with chemical composition. The progressive replacement of zinc by lead cation corresponds to a decrease in the ability of the cation to cross-link between phosphate chains. This induces a decrease in the elastic constant and also in the glass transition temperature and the softening point.