The K2NiF4 phases La1.5+xSr0.5-xCo0.5Ni0.5O4(+delta) (x = 0, 0.2) have been prepared by solid state reactions and structurally characterized by X-ray and neutron powder diffraction. The reduction behaviour, the magnetic properties and the electronic properties of these materials have also been examined. Oxygen hyperstoichiometry has been achieved in the lanthanum-rich phase La1.7Sr0.3Co0.5Ni0.5O4.08 with retention of the tetragonal symmetry. The excess oxygen occupies the ideal interstitial (0, 0.5, 0.25) sites of the tetragonal structure. The materials withstand reducing conditions (10% H-2-N-2) up to 800 degrees C via reduction of the B-site oxidation state to the divalent state (Ni2+/Co2+) and formation of oxide-ion vacancies within the equatorial planes of the structure. Formation of Ni1+ in these materials under reducing conditions is suggested to account for the oxygen stoichiometry and the magnetic behaviour of La1.5Sr0.5Co0.5Ni0.5O3.70. The electrical conductivity of the oxygen-rich semiconductor materials reaches a SOFC applicable limit (>100 S cm(-1)) at elevated temperatures.