This paper presents findings of a qualitative inquiry into international students' experiences and perceptions of their language use in an English-medium international university in multilingual Hong Kong, with particular attention to the use of English as a lingua franca (ELF) in relation to other languages. Based on data collected from in-depth interviews with a group of international students, the study found that the participants embraced a pluralistic conceptualisation of ELF. While they emphasised the importance of ELF for their academic and social integration in the university, the perceived resistance against the use of ELF by local students means that the language norms operating in the university cannot be pre-determined. It was also found that the participants expressed monolingual ideologies with respect to the use of ELF as a result of their concerns about social exclusion and linguistic disadvantage. Meanwhile, multilingual language practices and multilingual ideologies in relation to the use of ELF were also found in the participants' accounts of language experiences and perceptions. The findings also point to the tensions arising from the language choice between ELF and the local language, as well as the co-existence of contradictory language ideologies concerning monolingualism/multilingualism in the international university context.