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Changes in magnetization with temperature are of concern to the engineer designing permanent magnet devices. Of particular interest are the reversible and irreversible losses which occur when a magnet is heated to a slightly elevated temperature. In this paper the nature of the irreversible magnetization losses which occur when Co-Sm and other cobalt-rare-earth permanent magnets are heated above room temperature are discussed. It is shown that the losses are dependent upon the operating point of the demagnetization curve, and also that post-sintering thermal processing can play a significant role in controlling the changes in magnetization with temperature. Data are given on the effect of composition and high-temperature aging and cooling rate on the irreversible losses. Measurements of demagnetization curves at temperatures up to 200°C are given for a Co-Sm magnet and the data is related to the irreversible losses.