A simple phenomenological constitutive equation for porous materials is proposed, which allows a detailed description of irreversible compaction behavior at low pressures and reduces to the correct Hugoniot description at high pressures. The theory is compared to some existing data on Hugoniots of porous aluminum and iron, and fairly simple functional forms of the compaction relation are found to be adequate to fit the data. The constitutive relation is suitable for use with finite difference methods of solution of the one‐ and two‐dimensional equations of motion governing stress wave propagation. Examples of such solutions in one dimension are given to illustrate some of the features of the theory.