This investigation of English immersion learners explored processes of learning English, patterns of language use, and levels of comprehensibility among 34 predominantly Spanish-speaking 12th graders at the American School of Asuncion in Asuncion, Paraguay. The analysis of data from language learning histories, group interviews, perceived comprehensibility ratings, and questionnaires about language use led to the identification of linguistic dimensions affecting comprehensibility and of individual and sociocultural dimensions related to language learning and use. When judging comprehensibility, the native-speaker raters were influenced mainly by prosodic elements and overall fluency. Higher comprehensibility corresponded to girls as well as to students with some schooling in English language contexts, aspects of integrative motivation and more frequent L2 use. Findings were congruent with studies in immersion education, second language acquisition, sociolinguistics and social psychology.