Since the early 1990s, attention has been focused on the importance of self-determination in the education of students with disabilities. The purpose of this study was to further our understanding of the construct of self-determination by examining the relationship between the "Arc's Self-Determination Scale" and the "American Institutes for Research (AIR) Self-Determination Scale" student and educator versions. Using structural equation modeling, we found that the theoretical structure of the "Arc's Self-Determination Scale" and the "AIR Self-Determination-Student Scale" was supported by the data, while the proposed theoretical structure of the "AIR Self-Determination-Educator Scale" was not. The analyses suggested that each of the measures of self-determination was measuring a different aspect of the self-determination construct (i.e., it was not possible to create a higher order factor comprising each of the assessments); thus issues related to the goals of the research and the underlying theoretical perspective of each measure must be considered when determining the most appropriate measure of self-determination in research and practice. Implications and future research directions are discussed. (Contains 4 tables, 4 figures and 1 note.)