Contaminant migration through the vadose zone may be influenced by the presence of mineral colloids that are mobilized during infiltration events. In this work, we report model calculations and experimental data on the role of pore water ionic strength in the transport of silica colloids through water-saturated and unsaturated porous media. The transport model solves the advection-dispersion equation, together with kinetics equations for straining, air-water interface capture, and mineral-grain attachment. Parameters for air-water and solid-water interfacial area required by the transport model are derived through the application of a published liquid-vapor configuration model that accounts for dual-phase (i.e., air and water) occupancy of pore spaces. Comparison of experimental and modeled results reveals that the dominant colloid-immobilization mechanism transitions from straining, to air-water interface capture, and finally, to mineral-grain attachment as the ionic strength increases from 2 ¿ 10-4 M to 0.2 M. The results of this research provide a basic framework for interpreting how interactions between moisture content and pore water chemistry affect colloid mobility in the vadose zone.