The structure of the Cu/MnO interface has been studied using high resolution electron microscopy (HRTEM). Interfaces were formed by internal oxidation of a CuMn alloy. In the course of the reaction, MnO particles precipitate in several special orientations relative to the Cu lattice: “parallel” topotaxy, “twin” topotaxy, and a 55°[110] rotation yielding (111)Cu∥(002)MnO. Each of the three Cu/MnO orientation relationships has a characteristic particle morphology reflecting thermodynamically favourable interface structures. In parallel topotaxy MnO particles preferentially form flat {111}Cu/{111} MnO interfaces with a lattice mismatch of 21%. Although this mismatch is large, the existence of coherence strains in the Cu cannot be excluded. MnO particles in the 55°[110] orientation form regions of semi-coherent Interface where {200}MnO planes face a set of parallel {111} Cu planes with a mismatch of only 6%. This interface variant exhibits equally spaced steps, every 16 to 18 Cu {111} planes. Parallel to every step there is a misfit dislocation in the Cu at a stand-off distance of about 2 Cu {111} spacings. The relationship between structure and energy of the Cu/MnO interface is discussed.