According to the self-determination theory, when people have autonomous motivations, they show more interest and enthusiasm, which is manifested in their attitudes toward work. With a sample of 196 temporary agency workers (TAW), our main goal was to analyze over time and through structural equation models the relationships between motivations for being TAW (i.e., autonomous and controlled motivations) and work well-being (i.e., work engagement and burnout). In general, and according to what was predicted by self-determination theory, the results provide support for the importance of autonomous motivations for being TAW because they contribute to increasing work engagement over time. However, contrary to what was expected, both autonomous and controlled motivations seemed not to contribute to reducing or increasing, respectively, burnout over time. Surprisingly, the cross-lagged relationship between controlled motivations and work engagement was also nonsignificant. The practical implications of these findings for the management of temporary agency workers are discussed.