This paper draws on self-efficacy questionnaire data collected at the beginning and end of the TOEIC course, TOEIC scores and focus group discussions data from participating students at a technical university in Vietnam. While the quantitative data reveal positive effects in terms of TOEIC self-efficacy, the qualitative data show how changes in self-efficacy were moderated by the way students cognitively processed self-efficacy information. Results indicate there was a positive change in students' self-efficacy after joining the course. Self-efficacy was an important variable of students' TOEIC achievements and affected their achievement positively. Mastery experiences appeared to be the most influential source of TOEIC self-efficacy. Results are discussed as they relate to previous research and recommendations are made to include the development of self-efficacy in TOEIC instruction. The paper highlights that by targeting sources of self-efficacy, teachers can empower students with control over their own TOEIC ability development. Generally, TOEIC courses should promote learning environments that foster important sources of self-efficacy. Besides, course material and feedback should be delivered in a way that levels of self-efficacy are increased.