Total (electronic and ionic) and silver ionic conductivities of AgLaOS and AgLaOS + 0.15La(2)O(2)S were measured by the de four-probe method under a purified helium gas atmosphere at 323-723 K. The activation energy given by the silver ionic conductivity data of AgLaOS + 0.15La(2)O(2)S was 0.107 eV at 423-723 K. The silver ionic transport numbers of AgLaOS and AgLaOS + 0.15La(2)O(2)S at 723 K, determined by the Tubandt method, were 0.066 and 0.48 respectively. The difference of the maximum and minimum values of x in AgxLaOS at 723 K, i.e., (x(max)-x(min)) at 723 K, determined by Wagner’s solid-state coulometric titration method, was 0.0023.