The open circuit magnetization of rectangular bars of various modern permanent magnet alloys has been measured at temperatures up to 550° C. A differential ballistic method has been used and, after magnetization at room temperature, both reversible and irreversible losses occur on heating. These losses vary with material and dimension ratio, and in an attempt to explain these variations demagnetization curves at 500° C have been measured. To avoid the irreversible losses, stabilization at a higher temperature is better than ageing in an alternating field.