The LaGa1-x-yCoxMgyO3-delta solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0 less than or equal to less than or equal to 0.10 at x = 0.60 and 0 less than or equal to y less than or equal to 0.20 at x = 0.35-0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa1-x-yCoxMgyO3-delta ceramics were calculated from the dilatometric data to vary in the range of 12.4-19.8 X 10(-6) K-1 at 300-1100 K. Doping La(Ga,Co)O3-delta solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa1-x-yCoxMgyO3-delta membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.