Solid solution formation has been found in the perovskite-type system LaGa0.85-xMg0.15(Nb0.33Mg0.66)(x)O3-delta (x = 0 -0.20). With increasing x, oxygen ionic conductivity of the perovskites decreases, whilst p-type electronic conduction at temperatures above 1000 K increases. Oxygen-ion transference numbers in air were determined by e.m.f and Faradaic efficiency methods, varying in the range 0.77-0.96, decreasing with increasing x and temperature. The electron transference numbers of the end-member phase, LaGa0.85Mg0.15O3-delta, do not exceed 0.08 at temperatures of 870-1170 K and oxygen partial pressures from 10 to 1.01x10(5) Pa. Thermal expansion coefficients of LaGa0.85-xMg0.15(Nb0.33Mg0.66)(x)O3-delta ceramic materials are independent of composition, varying in the range (10.0+/-0.2)x 10(-6) K-1. Values of the activation energy for the total electrical conductivity are 104-106 kJ mol(-1) at 670-1000 K, and 67-76 kJ mol(-1) at 1000-1200 K.