This study aims to contrastively examine Korean and international students' experiences of taking subject courses at a Korean university. Focusing on the viewpoints of the students, rather than central authorities, we attempt to reveal how language use and cultural factors are interpenetrated in the praxis of English-medium instruction (EMI). The data consist of questionnaire responses of 249 Koreans and 61 international students from non-English-speaking countries and qualitative interviews with 23 Korean and 9 international students. Korean participants were found to perceive the effect of English proficiency on achievement (ENG) to be significant, while their confidence in participating in activities in EMI (ACT) classes was the lowest. Whereas the international students did not relate ENG to other factors of EMI, Koreans perceived this factor to be relevant to all the aspects of EMI. During the interviews, international students, with a limited level of tolerance of Korean use, were found to experience frustration with the lack of interaction in their courses. We discuss the findings in terms of ownership of English and highlight how important it is for the Korean and international students to have a shared definition of the role of English as a lingua franca in an EMI classroom.