Many methods have been used to analyze carbonate rocks for calcite-dolomite content. One approach is based on difference in density of the two minerals. In this study, volumes of about 50-gm samples of carbonates are determined with an air comparison pycnometer, and densities calculated. The precision (standard deviation) of the determination is 0.0028. Replicate weighings, volume determinations, and density calculations are done within five minutes. Calcite-dolomite composition is found by reference to a calibration line constructed with calcite and dolomite as end members. The 95 percent confidence interval for an analysis is two percent. The density contributions of terrigenous material present should be taken into account. This method is sufficiently precise and accurate for most purposes. It has, in addition, the advantage of being simple, very rapid, and relatively inexpensive.