dc conductivity measurements have been made as a function of temperature and electric field in bulk amorphous Se, Se-Te, Se-Sb, and Se-Te-Ge, in order to identify the conduction mechanism and to study the effect of various dopants in Se on the conductivity. It is found that the conduction in all the samples, except in Se-Te-Ge, is through the tail of the localized states via thermally activated tunneling. In the Se-Te-Ge system, the conduction in the high-temperature region is through the extended states and in the low-temperature region via thermally activated tunneling in the localized states. It is found that the electric field effect is to increase the conductivity and to decrease the activation energy. An empirical model has been suggested to explain these results.