Abstract: The guanidine hydrochloride method is an accurate and simple procedure for oxygen-18 determination in KH2PO4 and Ba3(PO4)2. The method is based on heating the samples with guanidine hydrochloride at 300℃. Two oxygen atoms per molecule of KH2PO4 or Ba3(PO4)2 are converted into CO2 which is then analysed by a mass spectrometer of model MAT-CH5. Only 5 mg of KH2PO4 or Ba3(PO4)2 is required for each determination and reproducibility of assays is better than±1 %.