Past studies on doctoral education have mostly focused on the critical role of supervisors in students' research experience. However, the role of peers has been relatively neglected. It is not clear how both supervisors and peers contribute to doctoral students' research experience. One possibility is that they are additive, which means that both supervisors and peers make distinct contributions to skill development and research satisfaction ("additive hypothesis"). The other possibility is a compensatory relationship, where low levels of supervisory support could be compensated for by the presence of peers ("compensatory hypothesis"). This study addressed this question using an explanatory sequential mixed methods design. In Study 1 (N = 421), structural equation modeling was conducted to examine which of the two hypotheses was supported. Quantitative results supported the additive hypothesis, indicating that both supervisors and peers were important in developing students' skills and enhancing satisfaction with the research experience. Study 2 was a qualitative study that involved interviews with 10 PhD students. The qualitative findings provided a nuanced understanding of the additive hypothesis and demonstrated the importance of research support and socio-emotional support provided by supervisors and peers. Overall, this study demonstrated that the greatest benefits would emerge when both quality supervision and peer support are present. The identification of factors affecting supervision and peer support shed light on the future directions for institutions, supervisors, and doctoral students to enhance the quality of doctoral education.