It has been known for some time that particles of nickel oxide of less than about 100 nm in size show superparamagnetism that increases as the particle size decreases. The origin of the particle magnetic moment responsible for this behavior has never been fully explained. This research indicated that the size of the particle rather than the presence of nonstoichiometry or impurities of reduced nickel determines the moment. The critical experiment was the measurement of magnetization versus magnetic field for a sample of nickel oxide prepared under conditions that preclude metallic nickel. Almost identical results were found for the original sample, which was black in color and thus nonstoichiometric, and after mild reduction in hydrogen at 400 K, which produced stoichiometry and changed the color to green. The magnetic susceptibility was inversely proportional to the particle size for a given method of preparation. This is consistent with a simple model of incomplete edges on the bounding planes of the crystallite and provides a possible basis for a practical method for measuring particle size in nickel oxide-containing samples.