Heat flow, seismic, and gravity data are used to determine the pressure and temperature at the crust/mantle boundary and at the Conrad seismic velocity discontinuity (where the compressional wave velocity jumps to 6.5 km/s or more) for 12 sites located in the central United States or in the High Plains region. If these discontinuities correspond to phase changes in thermodynamic equilibrium, the pressure and temperature are related by the integrated Clausius-Clapeyron equation. An 'empirical Clausius-Clapeyron' equation is determined as the best fitting line to the pressures and temperatures obtained at the different sites. Two thermal models of the crust corrseponding to different assumptions on the variation of the thermal conductivity, yield as an integrated Claussius-Clapeyron equation P = (0.085T+6.2) kbar and P = (0.095.T+4.8) kbar for the Moho (where T is the temperature in degrees Celsius). The correlation coefficient between the temperatures and pressures is too low to give strong support to the hypothesis that the Moho in this region corresponds to a phase change in equilibrium; however, the authors favor the hypothesis because the pressures and temperatures are within the range experimentally determind by Ito and Kennedy <1970> for the transformation from garnet-granulite to plagioclase-eclogite.