The x‐ray line broadening method of determining particle size was compared with direct measurement on electron micrographs. By controlled heating of the carbonate, magnesium oxide particles were prepared from 50 to 1000A in diameter. Particle size calculated from x‐ray data taken on a Geiger counter spectrometer agreed to ±10 percent with the microscope measurements. Mechanical mixtures of two different sizes were examined by the x‐ray method, but the particle sizes could not be determined unless the two maxima of the distribution curve were completely resolved.