The object of this study was to analyze quantitatively the powdery composition of phosphate-bonded investments used in routine clinics. It was well known that α-cristobalite or α-quartz or both of them were predominantly contained in phosphate-bonded investment powder. At first, seven synthetic mixtures of α-cristobalite and α-quartz were prepared. A standard quantity of β-Sn was then added as the internal standard to each of these mixtures. These mixtures were then investigated by X-ray diffraction. The intensities of the maximum peaks, which corresponded to α-cristobalite, α-quartz and β-Sn, were picked up in each X-ray diffraction pattern of the synthetic mixture. By using the intensity of β-Sn as the standard, both α-cristobalite and α-quartz were determined quantitatively, that is to say, the intensity (the weight of each maximum peak) was converted into the weight percentage. By using this relationship, the following results were obtained; for Ceramigold, 35±3% α-cristobalite and 46±3% α-quartz; for Ceravest, 50±2% α-cristobalite and 22±3% α-quartz; for Tai-Vest, 5±0.6% α-cristobalite and 84±6% α-quartz; for Biovest, 67±2% α-cristobalite and 3-1-1% α-quartz.