Perovskite-type LaGa0.65Mg0.15Ni0.20O3-delta exhibiting oxygen transport comparable to that in K2NiF4-type nickelates was characterized as a model material for ceramic membrane reactors, employing mechanical tests, dilatometry, oxygen permeability and faradaic efficiency measurements, thermogravimetry (TG), and determination of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10(-15) Pa to 40 kPa. Within the phase stability domain which is similar to La2NiO4+delta, the defect chemistry of LaGa0.65Mg0.15Ni0.20O3-delta can be adequately described by the ideal solution model with oxygen vacancies and electron holes to be the only mobile defects, assuming that Ni2+ may provide two energetically equivalent sites for hole location. This assumption is in agreement with the density of states, estimated from thermopower, and the coulometric titration and TG data suggesting Ni4+ formation in air at T < 1150 K. The hole conductivity prevailing under oxidizing conditions occurs via small-polaron mechanism as indicated by relatively low, temperature-activated mobility. The ionic transport increases with vacancy concentration on reducing p(O-2) and becomes dominant at oxygen pressures below 10(-7) 10(-5) Pa. The average thermal expansion coefficients in air are 11.9 X 10(-6) and 18.4 x 10(-6) K-1 at 370-850 and 850- 1270 K, respectively. The chemical strain of LaGa0.65Mg0.15Ni0.20O3-delta ceramics at 1073-1123 K, induced by the oxygen partial pressure variations, is substantially lower compared to perovskite ferrites. The flexural strength determined by 3-point and 4-point bending tests is 167-189 MPa at room temperature and 85-97 MPa at 773-1173 K. The mechanical properties are almost independent of temperature and oxygen pressure at p(O-2)= 1-2.1 X 10(4) Pa and 773 - 1173 K. (c) 2005 Elsevier B.V. All rights reserved.