Apart from advection and diffusion/dispersion, other physical and chemical processes can affect the movement of solute through a porous medium. The classical advection-dispersion equation does not usually model adequately the breakthrough curves resulting from such effects. The two-region model (TRM) is an attempt to model succinctly and easily the effects of physical and/or chemical nonequilibrium. Physical nonequilibrium is the focus of this paper. The additional parameters appearing in the TRM are the ratio of mobile to total pore fluid, β, and the apparent transfer rate of the solute between the mobile and the immobile regions, α. Meaning is ascribed to these parameters by identifying the various ways in which physical nonequilibrium can arise. An examination of published data shows that the dominant trend is a linear variation of the transfer rate α with mobile fluid velocity Vm . In order to develop an approach incorporating all porous media types, timescales of solute transport are identified and compared to the mass transfer timescale (1/α). It was found that the local advection timescale best characterizes the mass transfer timescale. Two trends were observed for the mobile water fraction β. For aggregated/saturated porous media, β was found to be constant or decreased with increasing pore water velocity, while for partially saturated soils, β was constant or increased with increasing moisture content.