K2NiF4 type oxides have received considerable attention in recent years as possible mixed conducting membranes for technologically important applications such as solid oxide fuel cells and oxygen generators. Much of this interest has focused on the oxygen interstitial containing oxides such as La2NiO4+δ. This contribution will review some of the recent work on these and related materials and will present new data on the La2MO4+δ (M= Ni, Co or Mn) systems. In particular data on the oxide ion diffusivity of these systems, as determined by oxygen isotopic exchange and SIMS analysis highlighting the fast oxide ion diffusion (1 x 10-7 cm2s-1 at 750oC) present, will be discussed and the significance of these data evaluated in relation to solid oxide fuel cell systems. A further section of the presentation will consider the oxygen defect species present and discuss the nature of the mobile oxygen ions. Finally some preliminary data on the application of these materials as cathodes will be presented and the performance of the cathodes on intermediate temperature electrolytes discussed.