Performance reference compounds (PRCs) are often used in passive sampling devices in order to allow sampler deployment for times shorter than would be required for full equilibration. They are especially important for in situ passive sampling of sediment beds, where equilibration of sampler and sediments could take years. In order to translate sampler concentration data, for both PRCs and target chemicals, into useful information such as water concentrations or chemical activities, an accurate mass-transfer model for the system is necessary. Here, a two-phase diffusion model for a polymer sheet in porous media is described and its use demonstrated. A method for calibrating sampler/site-specific mass transfer behavior using three PRCs and the model is described. The accuracy of such results are compared to measured porewater concentrations of seventeen target polycyclic aromatic hydrocarbons in a test sediment. Finally the diffusion model is exercised to predict how air-filled pores would affect deployment times when using polyethylene to sample contaminants in soils.