The properties of calcium phosphates of biological interest depend strongly on their calcium/phosphorus atomic ratios (Ca/P). Therefore, the precision for the determination of this ratio is crucial. We have investigated the accuracy of quantitative X-ray diffractometry (QXRD) analysis for the determination of Ca/P in compounds with Ca/P ranging from 1.5 to 2. For 1.5 < Ca/P < 1.667, a high accuracy is obtained with relative uncertainties on Ca/P values between 0.1% and 0.4%. In this domain of composition, QXRD is more accurate than classical wet methods. For 1.667 < Ca/P < 2, the precision is lower and wet chemical methods appear to be superior.