Colloids may transport contaminants through unsaturated soils to the groundwater, and colloid mobilization is associated with transient hydrological events and sustained, steady flow. We examined mechanisms of colloid mobilization from an intact, unsaturated soil core during relatively steady flow. We collected the core from a site where soils contain 26% clay and infiltration occurs only at capillary-pressure heads above −20 cm. We measured the colloid-mass flux during consecutive, 1.5-mo periods (P1, P2, and P3) distinguished by capillary-pressure heads (ΨB) of −18.5, −11.5, and −18.5 cm, respectively. Mean mass flux values were 0.0886, 0.197, and 0.171 mg h−1 for P1, P2, and P3, respectively. Intervention analysis showed a significant increase in the mass flux of 0.079 mg h−1 as ΨB became less negative. Results indicate that the number of soil pores through which water flows has a greater influence on colloid mobilization than do shear forces associated with elevated pore water velocities. Thus, colloid mobilization most likely is affected by a diffusion-limited, colloid-supply mechanism dependent on the number of pores contributing to flow.