Oxygen non-stoichiometry and electrical conductivity of the Pr2-xSrxNiO4 +/- 6 series with x=0.6-0.5 were investigated in Ar/O-2 (pO(2)=2.5 to 21000 Pa) within a temperature range of 20-1000 degrees C. The equilibrium values of oxygen non-stoichiometry and electrical conductivity of these nickelates were determined as functions of temperature and oxygen partial pressure (pO(2)). The nickelates with x=0-0.5 appear to be p-type semiconductors in the investigated temperature and pO(2) ranges. The nickelates with x=0.3-0.5 show very feebly marked pO(2) dependencies of the conductivity. Pr1.7Sr0.3NiO4 +/-delta shows the anomalies of the conductivity versus oxygen partial pressure which can be related to the orthorhombic-tetragonal crystal structure transformations. The conductivity of the Pr2-xSrxNiO4 +/-delta samples correlates with the average oxidation state of the nickel cations. The samples with x = 0.5 have the highest nickel oxidation state (approximate to 2.5+), the highest [Ni3+]/[Ni2+] ratio close to 1 and show the highest conductivity (approximate to 120 S/cm) in the whole pO(2) and temperature ranges investigated. (c) 2006 Elsevier B.V. All rights reserved.