[en] Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2-xSr_xNiO_4-δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20-1050 deg. C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating