The internationalization of European higher education and the corresponding proliferation of international Master's degree programmes are creating plurilingual educational environments that potentially enhance linguistic diversity. However, there is concern that the focus on English as the sole medium of instruction in such programmes could have the opposite impact on linguistic diversity, and could even reduce students' academic competence in their mother tongue. These issues are examined in our article from the viewpoint of 60 international Master's students at the University of Helsinki, who responded to an electronic questionnaire during 2011-2012. Here, we focus on the responses to questions concerning why the students applied to study in an international Master's programme in Finland, what challenges, advantages and disadvantages they perceived in doing so, and whether they felt linguistically able to cope with their studies. Furthermore, we present students' attitudes towards learning new languages, and their perceptions of how the use of English as the sole medium of instruction in international Master's programmes might influence linguistic diversity in general.