High-resolution electron microscopy shows the coexistence of several distinct core structures for coincident-site lattice boundaries in NiO bicrystals near ..sigma.. = 5, (310), and ..sigma.. = 13, (510). Lattice expansion perpendicular to the ..sigma.. = 5 grain boundaries is considerably smaller than calculated theoretically. Structural units in boundaries near ..sigma.. = 13, (510) exhibit translations normal to the grain boundary plane, resulting in a rumpling of the interface on an atomic scale. Our observations suggest the presence of a high concentration of Schottky pairs in the grain boundaries.