Although several theories, such as Conservation of Resources (COR) Theory, point to the potential of gain spirals of personal resources (i.e., the increase in personal resources over time), the processes that drive these spiral effects are under-theorized. Moreover, rigorous research that adequately tests these processes is lacking. Using two multi-wave studies (NStudy 1 = 121, NStudy 2 = 131), we examine a gain spiral comprising two central personal resources, namely self-efficacy and engagement. Specifically, by integrating self-determination theory into COR theory, we propose that intrinsic motivation represents a relevant process underlying the self-efficacy-engagement relationship and that engagement then fuels future self-efficacy. We find support for the mediation proposition in both studies. Furthermore, our results support a gain spiral of personal resources consisting of self-efficacy, intrinsic motivation, and engagement in Study 2. Our study enriches the theoretical understanding of the relationship between self-efficacy and engagement and makes an important empirical contribution by elucidating the dynamics of gain spirals.