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, &bgr;, and the apparent transfer rate of the solute between the mobile and the immobile regions, &agr;. 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 &agr; 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/&agr;). It was found that the local advection timescale best characterizes the mass transfer timescale. Two trends were observed for the mobile water fraction &bgr;. For aggregated/saturated porous media, &bgr; was found to be constant or decreased with increasing pore water velocity, while for partially saturated soils, &bgr; was constant or increased with increasing moisture content. ¿ 1998 American Geophysical Union