The solubility of three samples of pure tricalcium phosphate and of one sample of bone meal in 2% citric acid solution, in the proportion of 1:200, is almost total, that is,their content in P2O5. By other side, the rock phosphates present a partial solubility in 200 ml of 2% citric acid. The number of milliequivalents of hydrogen in 200 ml of 2% citric acid solution is almost the doble of the number required for the solubilization of 1 gram of phosphate or calcium fluophosphate. On the basis of the data obtained from this study it is possible to stablish the proportion of 1:200 for the extraction of P2O5 of rock phosphate and to interpret the extracted P2O5 as equivalent to tricalcium phosphate (TFE).