The structural and chemical stabilities, electrical conductivity, and thermal expansion of the A(2 - a)A(a)'MO4 - x oxides (A = La; A'= Sr; M=Mn, Fe, Co, Ni) with the perovskite-related K2NiF4-type structure were investigated and compared with the characteristics of perovskite-type oxides AMO(3 - x) containing the same cations. The K2NiF4-type manganites, ferrites, cobaltites and nickelates are assumed to be reduction products of the corresponding perovskite-type oxides. The thermodynamic stabilities, in terms of reversible oxygen desorption, were higher than those of the corresponding perovskite-type oxides. Within the range of oxygen partial pressure (pO(2)) from air to argon/H-2/H2O, the oxidation states of the M cations were determined. The comparison of the oxidation states of M in AMP(3 - x) and (AMO(3 - x)).AO gives evidence on the stabilizing influence of the AO interlayer on the perovskite layer. The electrical conductivity of the A(2)MO(4) oxides was of p-type and reached values close to 100 S cm(-1) at high oxygen partial pressures and 800 degreesC for nickelates and cobaltites. The thermal expansion of K2NiF4-type oxides is generally lower than that of the comparable perovskite-type oxides.