The observation that X-ray methods currently in use for determination of calcite and dolomite in rocks and sediments often give inconsistent results led us to structure an experiment to evaluate their relative accuracy. Three dolomite standards were selected from which 33 mixtures (11 each) of calcite and dolomite were prepared in triplicate (99 total). The intensities of the major calcite and dolomite reflections of each slide were measured by four methods: by fixed-time count on the peak maximum, integral count of a fixed interval across the peak, ruler measurement of peak height and planimeter measurement of peak area. The parameter "dolomite/dolomite plus calcite" was computed for each diffraction pattern. A curve of best-fit and the standard error of this parameter, as a function of weight percent dolomite, were computed for various combinations of individual and average data to determine the relative accuracy of the several types of intensity measures. Results of previous investigators were compared with our data to estimate their relative precision. It is concluded that X-ray peak-height (fixed time) intensity measurement provides the most rapid and precise procedure for determination of calcite and dolomite. Composite data for all types of measurement yield a calibration curve which gives results in terms of weight percent dolomite of about + or -6 at the 95% confidence level. Its true accuracy is probably better. Replicate analysis of unknowns yield mean values with standard deviations of about 3, expressed as percent dolomite. Most of the curves presented by prior investigators compare well with ours; that is, they lie within our documented confidence interval, and data derived from them can probably be considered reliable within the limits of accuracy established by this study.