The current inversion of pressure-particle velocity data for release from a high-pressure shock state to a pressure-density path usually depends critically upon the assumption that the release processes is isentropic. It has been shown by Kieffer and Delaney that for geological materials below stresses of ~150 GPa, the effective viscosity must be ≲103 kg m-1 s-1 (104 P) in order that the viscous (irreversible) work carried out on the material in the shock state remains small in comparison to the mechanical work recovered upon adiabatic rarefaction. The available data pertaining to the offset of the Rayleigh line from the Hugoniot curve for minerals, the magnitud of the shear stress in the high-pressure shock state for minerals, and the direct measurements of the viscosities of several engineering materials shocked to pressures below 150 GPa yield effective viscositie of ~103 kg m-1 s-1 or less. We infer that this indicates that the conditions for isentropic release of minerals from shock states are achieved, at least approximately, and we condlude that the application of the Riemann integral to obtain pressure-density states along the release adiabats of minerals in shock experiments is valid.