Six glasses of the chemical composition 10ZnO-xPbO-(90-x) P2O5 were prepared. With an increase in PbO content a non-monotonous step like increase in the density, in the glass transition temperature, and in the refractive index was observed. From the Raman and IR spectra studied the evidence is given for the phosphate network depolymerization as PbO content increases. Increase in PbO content leads also to an increase in refractive index (n) up to n = 1.74, for x = 55, and to an increase in the glass transition temperature (T-g) from T-g = 270 degrees C (x = 30) to T-g = 360 degrees C (x = 55).