The study presented in this article explores prisoners' academic motivation structure from the theoretical perspective of self-determination theory, using the Academic Motivation Scale (AMS). Analysing survey responses from 529 (29 female, 500 male) prisoners with Norwegian citizenship who participated in education while being incarcerated, the authors investigate how prison students' motivation might be "reduced" or summarised using a smaller set of factors or components than extant studies. A confirmatory factor analysis suggested that a five-factor model, including intrinsic motivation, three types of extrinsic motivation (namely "identified regulation," "introjected regulation," and "external regulation") and amotivation, yielded the best fit with the data provided by the prisoners. An alternative three-factor model created by collapsing the three extrinsic dimensions into a single dimension was found to fit the data poorly. The structural model revealed that younger prisoners displayed more controlled academic motivations than older ones, who displayed more autonomous motivations. Contrary to the authors' expectations, prisoners with a higher level of education did not display more autonomous academic motivations than those with a lower level.