This paper reports on theptontransition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range300 K⩽T⩽315 K,dc conductivity in the temperature range100⩽T⩽300 K,and optical band gap(Egopt)have been carried out forSe75In25−xPbx(x=0,5,10,15)samples. Thep-ntransition occurs with very low addition of Pb impurity (5 at. &percent;). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads ton-type conduction in these glasses. ©1997 American Institute of Physics.