While learners of a second language (L2) are increasingly interacting in small groups as part of a communicative methodological paradigm, very few studies have investigated the social dynamics that occur in such groups. The aim of this study is to introduce a group work dynamic measuring instrument and to investigate the relationship between group work dynamic (GWD), or the social climate existing within a group, and learners' state-level motivational responses and amount of language produced in interactive tasks. The context for the classroom-based study was a speaking-skills course consisting of a small group of 10 Korean English learners of intermediate to high intermediate proficiency level enrolled in a Korean university TESOL certificate program. The data were based on 15 different tasks that were part of the course and two work groups for each task for a total of 30 groups. Instruments of data collection consisted of audio-video recordings, transcription of all verbal and nonverbal language produced in the tasks, the GWD measurement instrument, and post-task motivation questionnaires. Results from correlational analysis showed a significant relationship between GWD and task motivation and between GWD and language production. Nonverbal-related behaviors, furthermore, were found to be particularly influential in shaping GWD and were significantly associated with learners' task motivation, especially in the form of greater task enjoyment and sense of success. The study's findings therefore add support to socio-contextual perspectives in L2 motivation and applied linguistics that emphasize the critical role of social and group factors on motivation and language development and imply a need for educators to focus on creating positive group dynamics in their classrooms.