This paper outlines the findings of a mixed-method study of private supplementary tutoring received by students at international schools offering the International Baccalaureate Diploma Programme (IBDP) in China. Phase One was an online survey of 151 IBDP graduates across 14 schools, while Phase Two included semi-structured interviews with school administrators, teachers, and students in five IBDP schools. Almost one quarter (23.8%) of surveyed graduates from the sample of IBDP schools reported having received private supplementary tutoring. A hierarchical regression analysis indicated that participation in private supplementary tutoring was negatively associated with final IBDP scores. Interview data illuminated that private supplementary tutoring was discouraged by most teachers and administrators due to a perception of tension with the IB's educational philosophy, a view that private supplementary tutoring is not conducive to IBDP assessments, and that providers lacked specialised knowledge of the IBDP curriculum. Nevertheless, interviewees noted that low performing IBDP students often utilised private supplementary tutoring for remedial purposes, especially for Mathematics and languages.