A model is presented for analyzing experimental observations of high-pressure phase equilibria among two crystalline polymorphs. The equilibrium phase boundary is determined in the quasi-harmonic approximation as a function of three parameters: the zero-kelvin transition pressure (P0 tr ), the average vibrational frequency of one of the phases (ω1), and the ratio of the average frequencies of the two phases (ω2 /ω1 ). Thus modeled, the phase boundary can be extrapolated from the moderate to high temperatures at which experiments are carried out to low temperatures, at which quantitative comparisons with quantum mechanical calculations are possible. A kinetic analysis is presented in terms of time-pressure-transformation relations. With suitable assumptions, this analysis permits an average activation energy for the transformation (Δg¿ ) to be determined from the hysteresis observed in pressure-temperature phase equilibrium studies. ¿American Geophysical Union 1987