Understanding the mechanisms controlling colloid transport and deposition in the vadose zone is an important step in protecting our water resources. Colloid transport in unsaturated porous media was studied using X-Ray Microtomography (XMT), which is a non-destructive imaging technique that provides three-dimensional images at a resolution on the order of 3 μm/pixel. Under favorable attachment conditions, 5 -15 μm hydrophobic silver coated hollow glass spheres were suspended and drained through sintered glass bead columns at flow rates of 0.5, 5.0 and 50.0 mL/hr. The results show that total colloid deposition was dependent on drainage flow rate and saturation. A positive correlation was found between the total colloid concentration and the number of trapped air bubbles. An image processing algorithm was written to assess the partitioning of colloids to the solid-water interfaces (SWIs), air-water interfaces (AWIs), and air-water-solid (AWS) contact lines, which led to the conclusion that at medium to high water contents, AWIs and AWS contact lines were equally important for colloid attachment and mobilization. Lastly, colloids were shown to be left behind the main drainage front, becoming immobilized in the disconnected wetting phase by attachment to SWIs, or in pendular wetting phase rings.