The structure of an LiNi0.4Fe0.6O2 cubic solid solution is determined using magnetic measurements and electron diffraction. It is found that this solid solution has a microinhomogeneous structure due to the formation of superparamagnetic clusters. The electron diffraction analysis of LiNi0.4Fe0.6O2 samples has revealed diffuse scattering characteristic of the substitutional short-range order in ordered solid solutions with a B1-type structure. It is shown that the short-range order is associated with the LiNiO2-type rhombohedral superstructure (space group ), i.e., with the redistribution of lithium and nickel atoms in the (111)B1 alternating planes. The short-range order is observed in regions with a nickel content higher than the mean nickel content corresponding to the macroscopic composition.