Solid state electrochemical cells have been applied to oxidize Pt-Ni alloys and NiO in contact with Pt. The resulting interfaces have been analyzed by analytical transmission electron microscopy. Highly oxidized phases and complex morphologies have been identified on a nanometer scale in both cases. These phenomena are discussed in the context of solid state pattern formation and characteristic reactions of the system Pt-Ni-O, such as alloy oxidation and reactions between NiO and Pt. Reaction and diffusion mechanisms, which can explain the formation of the complex microstructures, are introduced for each case.