Vibronic spectra of V2+ and Cr3+ impurities in MgO have been used to determine the pressure dependences of the energies of MgO phonons at several critical points of the Brillouin zone to more than 100 kbar at 90 and 295 K. From these data, approximate densities of states for MgO at 100 and 500 kbar and vibrational partition functions were constructed, and contributions to the constant-volume heat capacitiy, entropy, internal and free energies, and the Debye temperature were calculated. Argon was successfully used as a nearly hydrostatic pressure medium for the measurements of 90 K to 102 kbar.