The room temperature static compression of CaO has been measured in a diamond anvil cell to 1.35 Mbar. The compression of the B1 phase and the volume change at the B1 to B2 transition agree with previous diamond cell results. The B2 phase has been observed down to 527 kbar and seems slightly more compressible at room temperature than deduced from shock wave experiments. Eulerian as well as semiempirical equations of state account well for the data and yield similar bulk moduli for both phases as a function of pressure. Equation-of-state parameters are K0=1.11¿0.01 and 1.3¿0.2 Mbar and K'0=4.2¿0.2 and 3.5¿0.5 for the B1 and B2 phases, respectively, and V0=.4.8¿0.2 cm3/mol for the B2 phase. For a pressure of 550 kbar, at which no data extrapolations are required, one finds K=3.04 and 2.96 Mbar with K' values of about 3.0 and 2.5 for the B1 and B2 phases, respectively. Recent theoretical structure calculations predict room temperature compressions in fair agreement with the static results. ¿ American Geophysical Union 1988