The short-term response of a univariant or divariant isochemical phase boundary to a pressure change is studied. The pressure change can be caused by vertical displacement of material in response to surface loading. The aim is to determine if the density discontinuity is nonadiabatic, i.e., will affect fluid flow, or adiabatic. For the basic state both a quasi-static mantle with a smooth thermal structure in the transition region and the case of a steady convection current through the boundary are considered. In the latter case the peculiar temperature distribution near the boundary affects significantly the response to additional loading. A divariant transition behaves fully nonadiabatic at low convective velocities below the ''Verhoogen limit'' but increasingly adiabatic above. A divariant transition behaves partly adiabatic already in a static mantle, and the adiabatic character is strongly enhanced with increasing velocity of convection. With realistic combinations of the relevant parameters for a divariant mantle phase change the response will be hardly more than 10% nonadiabatic, and probably less than 1%. This result affects especially the interpretation of the gravity anomaly above formerly glaciated areas in terms of mantle viscosity and layering of mantle convection.