The system LaMn1−xNixO3+&lgr;has been prepared under varying conditions for the entire range of compositions 0≤x≤1. The saturation magnetization, lattice parameters, and chemical analysis have been measured for each sample. The origin of discrepancies in the literature on the crystal structure of LaMnO3is shown to be due to different preparation procedures which result in the presence of differing amounts of Mn4+: the lattice symmetry changes from orthorhombic to rhombohedral as the Mn4+concentration exceeds 21% of the total manganese. The system is ferromagnetic with magnetization only slightly less than the spin‐only value over the compositional range 0.2≤x≤0.5: a ferromagnetic Ni3+&sngbnd;Mn3+interaction is indicated. The system remains orthorhombic throughout the range 0≤x<0.5; it shows an increasing distortion to an apparently monoclinic symmetry in the range 0.5<x≤0.8. LaNiO3is rhombohedral at room temperature. The lattice symmetry is compatible with Ni3+in a low‐spin state: this is also in accord with the magnetization values. Qualitative concepts used to interpret the system (La,Ca)MnO3are applied to interpret the magnetic and crystallographic properties of this system as well as the magnetic properties of some rhombohedral perovskite‐type systems rich in Mn3+ions.