The (metastable) tetragonal phase in 3-4 mol % Y2O3-ZrO2 alloys undergoes a transition to the monoclinic form in the 200-300-degrees-C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating, The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic half arrow right over half arrow left tetragonal transition in ZrO2-1 mol % Y2O3 alloy at 850-750-degrees-C and the same phase change in 2, 3, 4 and 6 mol % Y2O3 compositions at the eutectoid temperature of about 560-degrees-C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol % Y2O3 compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1-12 mol % Y2O3 region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.