Amid the increasing popularity of task-based language teaching, task motivation is considered one of the keys to success, but motivational influence among learners has not been sufficiently addressed in the literature. The purpose of this study is to investigate the process of motivation contagion in language learning pair work. Specifically, it tests the hypothesis that motivation contagion in pairs occurs either by observing the motivational orientation of the partner (i.e., priming), or as a result of the perceived expectations of the learning task (i.e., expectancy formation). A total of 140 Japanese university students worked in pairs on a picture description task. The analysis of the post-task questionnaires indicated that motivation contagion occurred both by priming and expectancy formation, but the impact of the latter was more significant. It was also found that highly motivated learners were strongly affected by expectancies about the quality of task involvement and interpersonal relations, while less motivated learners were influenced more by their partners' motivation levels. Through the learners' retrospective accounts, we also found that mutually supportive attitudes and behaviors play an essential role in boosting motivation contagion. The findings provide pedagogical implications for designing dyadic activities that can help learners motivate each other.