This study explored students' perceptions of instructional practices of international teaching assistants (ITAs) who did and did not participate in additional professional development through a Teaching Methods course. The study also sought to understand the components of the Teaching Methods course that supported ITAs in inquiry-based chemistry lab instruction. Participants were treatment ITAs (n = 4), control ITAs (n = 5), and students who were enrolled in a project-based guided inquiry (PBGI) introductory chemistry lab II course (n = 341). All ITAs attended a week-long course-specific training for all TAs, while treatment ITAs received additional professional development (PD) on teaching through a semester-long Teaching Methods course with all department TAs. Data on students' perceptions of their ITAs were collected using a postsurvey that included Likert-scale items and open-ended questions. We also collected written reflection assignments from treatment ITAs. Data were analyzed using quantitative and qualitative methods. Likert questions were analyzed using Wilcoxon signed-rank tests, and qualitative data from student surveys and TA reflections were analyzed using an inductive content analysis approach. Students with treatment ITAs (n = 160) qualitatively reported more positive perceptions of ITAs' instructional practices and learning outcomes than students of control ITAs (n = 181). Specifically, treatment ITAs were reported to be significantly more supportive, interactive, and effective than control ITAs (all p values <0.001). Treatment ITAs reported increased understanding of their role as facilitators of student learning in PBGI laboratories. Results also suggest that the Teaching Methods course, along with concurrent teaching experience, enabled treatment ITAs to increase their confidence in teaching PBGI laboratories and to provide a supportive learning environment to students. These findings imply that TAs can learn more about active learning approaches and how to support student learning in inquiry-based chemistry lab if they receive sustained instruction on learning theories and inquiry and engage in reflective practice. Results have implications for ITA training and on chemistry teaching and learning.