The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La 2 Ni 1 - x M x O 4 + δ (M=Co, Cu; x=0.1-0.2) with K 2 NiF 4 -type structure and perovskite-like La 0 . 9 0 Sr 0 . 1 0 Ga 0 . 6 5 Mg 0 . 1 5 Ni 0 . 2 0 O 3 - δ and La 0 . 5 0 Pr 0 . 5 0 Ga 0 . 6 5 Mg 0 . 1 5 Ni 0 . 2 0 O 3 - δ , were studied in the oxygen partial pressure range from 10 - 1 8 Pa to 50kPa at 973-1223K. Within the phase stability domain, the conductivity of layered nickelates is predominantly p-type electronic and occurs via small-polaron mechanism, indicated by temperature-activated hole mobility and p(O 2 ) dependencies of electrical properties. In oxidizing conditions similar behavior is characteristic of Ni-containing perovskites, which exhibit, however, significant ionic contribution to the transport processes. The role of ionic conduction increases with decreasing p(O 2 ) and becomes dominant in reducing atmospheres. All nickelate-based phases decompose at oxygen pressures considerably lower with respect to Ni/NiO boundary. The partial substitution of nickel in La 2 Ni(M)O 4 + δ has minor effect on the stability limits, which are similar to that of La 0 . 9 0 Sr 0 . 1 0 Ga 0 . 6 5 Mg 0 . 1 5 Ni 0 . 2 0 O 3 - δ . On the contrary, praseodymium doping enhances the stability of La 0 . 5 0 Pr 0 . 5 0 Ga 0 . 6 5 Mg 0 . 1 5 Ni 0 . 2 0 O 3 - δ down to p(O 2 ) values as low as 10 - 1 7 -10 - 1 0 Pa at 1023-1223K.