This study examined whether 30 learners of Japanese in the United States who engaged in a semester-long video-based eTandem course made gains in global language comprehensibility, that is, ease of understanding (Derwing & Munro, 2009), and what linguistic correlates contributed to these gains. Speech excerpts from Week 2 and 8 of tandem interactions were retrieved and later assessed subjectively and objectively for global comprehensibility and its linguistic correlates (lexical appropriateness, lexical richness, speech rate, and morphological accuracy) in a pre/posttest sample design. The results revealed that, although the group made significant gains in vocabulary and some gains in grammar, improvement in overall comprehensibility was subject to considerable individual variability. According to a follow-up cluster analysis and discriminant analysis, increase in speech rate was the strongest predictor of those individuals who improved comprehensibility. The findings suggest that telecollaborative interaction may promote the development of vocabulary and, to some extent, grammar, but that significant gains in comprehensibility come mostly from the fluency trait of speech rate and may require longer interactional intervention. The findings have implications for the design of telecollaboration that supports second language learning.