Migration of colloids such as viruses and bacteria and colloid-facilitated transport of strongly sorbing contaminants in the subsurface environment are important environmental concerns. In this study, batch and column experiments were conducted to examine the mechanisms of colloid retention and transport in sand media, either 100% hydrophilic or partially hydrophobic with mixed wettability. Latex microspheres (diameter 19 nm) were used and the experiments were run under constant ionic strength of 100 mM and pH of 7.5. Significant retardation in breakthrough of the latex particles was observed in the presence of hydrophobic sand and it became more significant as the fraction of hydrophobic sand increased. Strong partitioning of colloids onto the hydrophobic surface was likely the main mechanism contributing to the retardation. Batch experiments with hydrophilic sand, hydrophobic sand and the mixture of them are used to quantify the amount of particle sorption by the hydrophobic sand. In addition to hydrophobic interactions, the different transport behavior of the latex particles observed under unsaturated flow conditions between the hydrophilic and partially hydrophobic media may be attributed to the complex flow regime occurring in the medium with mixed wettability. Results from this study provide insights on colloid behavior in the vadose zone composed of media with heterogeneous surface properties.