An equation of state for porous mixtures is described based on the mixing properties of the Gibbs potential. For a mixture, neglecting surface energy, the Gibbs potential is simply the sum of the Gibbs potentials of the components. Nitrogen is added to account for porosity, with the mole number proportional to the porosity. The Gibbs potential for fluid species is calculated by means of a perturbation theory originated by Weeks, Chandler, and Anderson. A semi-empirical Debye-Gruneisen equation of state with a Murnaghan form for the zero degree isotherm is used to describe solids. Due to the complexity of the Weeks-Chandler-Anderson fluid equation of state, the possibility of using an ideal gas representation for nitrogen was investigated. A very good match to the shock Hugoniot for porous copper results from this approach. The model is also used to calculate the Hugoniot for an Al-TEFLON mixture and compared to data obtained by Miller and Lindfors and by Holt and Mock. ©2000 American Institute of Physics.