A method of X-ray diffraction, incorporating mass absorption, for the quantitative determination of calcite and dolomite (ferroan), using rock powder pellets of the Oxford Clay sediments, presented in this paper, has produced satisfactory results, Ratios of the peak heights for calcite at 29.43°, 2θ, and for dolomite (ferroan), at 30.84°, 2θ, peak positions obtained by X-ray diffraction, corrected for mass absorption and the values of carbon dioxide determined by dividing the total contents of calcite dolomite, by a factor of 2.2, have been used in this method. The method, presented in this paper has been found fairly rapid and produces accurate and reproducible results.