This research employed a case study to examine motivational factors which influenced two learners' participation in course-adjunct English activities. The first participant was a high-motivation learner who joined many of the activities, while the second one was a low-motivation learner who rarely attended the activities. An in-depth interview was conducted with each of them to investigate the cases, and the interviews were described narratively. Three main motivational factors based on self-determination theory: autonomy, competency, and relatedness, were used as the framework for the analysis. The findings revealed that the high-motivation learner was clearly driven by intrinsic motivation, a sense of autonomy, mastery experiences in improving her own English proficiency, and feelings of relatedness in the learning context. The low-motivation learner, on the other hand, did not show a sense of autonomy. For him, scores were the main motive for completing the activities. If they had not been compulsory, he would not have joined the activities as he did not experience feelings of relatedness in the learning context. The findings implied that the two learners' motivational factors played a crucial role in their decision of whether or not to participate in course-adjunct English activities.