The identification and discrimination of cubic (c) and metastable t'-zirconia phases by conventional X-ray diffraction has proved to be a difficult problem because of the crystallographic similarities of the two phases. In this study, the c-phase and t'-phase in biphasic metastable zirconia samples have been successfully distinguished by S-ray diffraction; data mere collected using a high-resolution powder diffractometer with monochromatic radiation and the peak shapes were fitted using a peak-fitting program to distinguish the different phases. From this study, the minimum temperature, T-0, at which the t'-phase can be obtained on cooling rapidly compositions in the range 3-7 mol% Y2O3-ZrO2 to room temperature was estimated as similar to 1425 degreesC. For a constant temperature above 1425 degreesC, the lattice parameters of the t'-phase of 3-7 mol% Y2O3-ZrO2 compositions within the two-phase region of the phase diagram were unaffected by the yttria content; only the amount of t'-phase formed was affected by the yttria content.