The study examines potential safety outcomes of communication difficulties in mixed nationality crews on Greek and Norwegian cargo and passenger vessels. The aims are to examine the prevalence of unsafe situations due to language misunderstandings on vessels with different degrees of mixed nationality crews, and the factors influencing this. The study is based on quantitative survey data (n=367) and qualitative interviews (n=15) with seafarers on different types of Greek and Norwegian vessels with different degrees of mixed nationality crews. The survey indicates that respondents on board mixed-nationality vessels experience more unsafe situations because of language misunderstandings between different nationalities on board. Work pressure are also among the influencing factors. The qualitative data provide illustrations of such situations, including cultural differences on board.