The amount of second language (L2) use has significant influence on native speakers' comprehension of L2 learners' speech. Nonetheless, few empirical studies examine how differences in the amount of language use affect the intelligibility and comprehensibility of nonnative speakers' reading and spontaneous speech. This study aims to contribute to this gap. Participants included 100 adult Vietnamese native speakers, who were asked to read five sentences and a 100-word passage and provide a one-minute oral description of their daily activities. Then, 20 American students in a graduate TESL program at Northern Arizona University, U.S.A., rated the speech samples for intelligibility and comprehensibility. Results show that the degree of L2 use contributed to considerable variance in predicting performance ratings of only L2 spontaneous speech. This study provides evidence that variation in the amount of L2 use affects L2 intelligibility. This study also challenges the well-established view that English language use best predicts the degree of L2 perceived comprehensibility, regardless of nonnative reading ability or spontaneous speech.