Ultrasonic data for a high-purity single-crystal of hematite (α-Fe2O3) as a function of decreasing temperature at P=1 bar indicate a 10% decrease in the bulk modulus K across the second-order Morin magnetic transition at Tm =261 K. The Morin temperature increases with pressure and occurs at 298 K near 30 kbar. Thus a theoretical pressure-volume trajectory for the isothermal compression of hematite at 298 K has a change in slope at P=30 kbar, reflecting the discontinuous decrease in K across the Morin transition. Static compression data for a single-crystal specimen and for a coarse-grained powder do indicate such a discontinuity in slope and agree very well with the theoretical P-V trajectory. In contrast, data for fined-grained powder or polycrystalline specimens do not reflect such magnetoelastic behavior, since they are single-domained and their elastic properties are not influenced by domain wall-stress interactions. Such discontinuities in the bulk modulus are difficult to observe in static compression experiments but are commonly associated with second-order phase transitions and may help to explain anomalous compression data for other magnetic oxides.