In the current study we set out to investigate source language interference in the visual modality (in sight translation – ST) and in the auditory modality (in simultaneous interpreting – SI). We probed interpretations of cognates, interlingual homographs and passive structures in single sentence contexts as performed from English to Polish by 47 advanced interpreting trainees. We also analysed temporal measures: ear-voice span (in SI) or eye-voice span (in ST) as well as total translation time. The results showed a higher level of interference in ST in the case of homographs and a mixed pattern of results for the remaining measures. We also obtained interesting task-independent results, namely an 80% rate of global passive retention testifying to a high level of syntactic priming in both modes of interpreting. We discuss these results in the context of different types of interference occurring in interpreting and conclude that there might be a similar global level of interference in the two tasks, however with differing underlying patterns. This is the first study to date to directly compare interference levels between ST and SI in such controlled conditions. Our results contribute to the understanding of complex linguistic processes occurring across modalities in interpreting tasks.