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Purpose The purpose of this study was to investigate the effects of 2nd language proficiency and linguistic uncertainty on performance and listening effort in mixed language contexts. Method Thirteen native speakers of Dutch with varying degrees of fluency in English listened to and repeated sentences produced in both Dutch and English and presented in the presence of single-talker competing speech in both Dutch and English. Target and masker language combinations were presented in both blocked and mixed (unpredictable) conditions. In the blocked condition, in each block of trials the target–masker language combination remained constant, and the listeners were informed of both prior to beginning the block. In the mixed condition, target and masker language varied randomly from trial to trial. All listeners participated in all conditions. Performance was assessed in terms of speech reception thresholds, whereas listening effort was quantified in terms of pupil dilation. Results Performance (speech reception thresholds) and listening effort (pupil dilation) were both affected by 2nd language proficiency (English test score) and target and masker language: Performance was better in blocked as compared to mixed conditions, with Dutch as compared to English targets, and with English as compared to Dutch maskers. English proficiency was correlated with listening performance. Listeners also exhibited greater peak pupil dilation in mixed as compared to blocked conditions for trials with Dutch maskers, whereas pupil dilation during preparation for speaking was higher for English targets as compared to Dutch ones in almost all conditions. Conclusions Both listener's proficiency in a 2nd language and uncertainty about the target language on a given trial play a significant role in how bilingual listeners attend to speech in the presence of competing speech in different languages, but precise effects also depend on which language is serving as target and which as masker., Funding Agencies|Purdue University