Several models have been suggested to describe the volume behavior of porous materials under hydrostatic loading. The model presented here accounts for variations in both porosity and the amount of fluid contained within that porosity. The model also attempts to predict the one-dimensional strain loading pressure-volume relationship and the stress difference-confining pressure curve. Only the simplest assumptions were used for the development of this model. These assumptions are discussed and the predictions for Mt. Helen tuff are compared to experimental data.