Using Englishes as a lingua franca ("EsLF") or international language among EFL learners in an out-of-class setting induces anxiety, then debilitates the intelligibility and comprehensibility of speakers. This research project reports the conceptualisation of EsLF anxiety and its effects on intelligibility and comprehensibility, with the participation of 240 Thai and foreign graduate students in universities across Thailand. The Anxiety Scale for Spoken Englishes as a Lingua Franca (ASSELF) was developed and the results were used in an exploration of EsLF anxiety. The scale was validated with exploratory factor analysis (EFA), using principal axis factoring. The analysis indicated a three-factor solution including interlocutor-induced difficulties, language-processing difficulties, and apprehension over interlocutors. Independent samples "t" test pointed out a significant difference on how EsLF anxiety is experienced by Thai and foreign students. Further, the study examined the relationship of EsLF anxiety and the intelligibility and comprehensibility of international speakers. Correlational relationships revealed that (i) as anxiety increases, intelligibility and comprehensibility decrease; and (ii) as intelligibility increases, so does comprehensibility. Among the three factors of EsLF anxiety, apprehension over interlocutors debilitated the intelligibility of the international speakers significantly. Specific anxiety-provoking situations such as a proficient speaker and a native-like accented speaker are significant sources of unintelligibility and incomprehensibility. The research concludes with a discussion of limitations, and implications for English language education and future research.