Secondary data analysis using the National Longitudinal Transition Study-2 dataset was conducted to examine the degree to which autonomy, psychological empowerment and self-realization (three of four essential characteristics of self-determination) play a mediating role in the relationship between school-based factors and postschool outcomes. The results suggest that autonomy, psychological empowerment and self-realization play a large and significant role in mediating the relationship between school-based factors and postschool outcomes. With over 50% of the indirect effects (i.e., the aggregated effects of the school-based factors on the outcome constructs through the self-determination constructs) significant, this provides support for the critical role of self-determination in secondary transition services and supports. Implications for research and practice are discussed.