By using sound wave velocity data the entropy changes and dP/dT of 10 polymorphic transitions and solid-solid reactions have been calculated according to Debye theory. These values show systematic deviations from experimentally determined values of the same parameters. This data set allows the application of a correction factor to calculate values for cases where experimental determinations of the phase diagram or entropies are not available but where sound wave velocities are available or can be estimated for all phases. When applied to &ggr;Mg2SiO4 and &ggr;Mg2GeO4 disproportionation, the estimated values of dP/dT are +0.016 and +0.027, respectively. The entropy expressions indicate that negative dP/dT for a polymorphic transition implies either anomalously low sound velocities in the high-pressure phase as compared to the low-pressure phase or non-Debye contributions to the specific heat. This is not necessarily the case for disproportionation reactions, as illustrated by &ggr;Fe2SiO4 and &ggr;Mg2SiO4.