An X-ray diffraction technique is described for determining calcite-dolomite ratios with a standard deviation of 1.3%, and a detection limit of about 5%. Well-mixed powdered rock is mounted on a glass slide with thermo-setting plastic or Canada balsam and ground on a glass plate to produce a smooth surface. The slide is mounted in an X-ray diffractometer, and the pattern is recorded of the d spacings corresponding to 29 degrees to 31 degrees 2 theta using a scanning rate of 1/8 degrees per minute. The intensity of the d spacings of calcite and dolomite are determined by summation of gamma counts over the curves, or by planimeter measurement of the area under the curves. Calibration curves are made of weight mixtures of calcite and dolomite, and unknown mixtures are compared. A plot of weight percent dolomite (y) against area (sub dol.) + area (sub cal.) (x) for a series of standards gave a linear relation: y = 95.3x + 5.52.