OpenCourseWare (OCW) has received increasing attention over the past few years in higher education. These courses provide appealing opportunities to view classes taught in well-established universities worldwide. The current study aims to examine how OCW lectures can serve as authentic learning materials to facilitate vocabulary acquisition for learners learning English as a foreign language (EFL). It further investigates factors that influence the effectiveness of vocabulary acquisition via OCW lectures. Three open-course lectures from MIT and Yale University, on psychology, physics, and music, were selected. Sixty-five Taiwanese college students participated in the study. The results of pre- and post-testing indicate that learners were able to gain L2 vocabulary knowledge by simply viewing an OCW lecture once. Level of vocabulary (academic vocabulary, technical vocabulary and low-frequency vocabulary) was found to be the most influential factor affecting vocabulary acquisition through OCW lectures. In addition, learners improved more on technical words than on academic ones. Verbal elaboration was also an important factor affecting vocabulary acquisition; the more explicit the elaboration is the greater the vocabulary gain. Frequency of occurrence, on the other hand, had positive but relatively small effects on learners' vocabulary gain. (Contains 14 tables.)