This paper presents the preliminary results of an empirical study aimed at exploring the effect of previous preparation in the performance of simultaneous interpreting of specialised speeches, carried out with advanced undergraduate interpreting students. Simultaneous interpretation may be the most frequent modality of interpretation used in multilingual specialised and technical conferences, including international scientific conferences. The linguistic and extra-linguistic characteristics of specialised speeches impose additional demands on comprehension processing. Previous preparation is here understood as an instance of acquisition of prior topic knowledge, which has been observed to support the comprehension of scientific discourse. A within-subject design was used to test the effect of an instance of previous preparation on neutral and difficult segments of a scientific speech. Ear-voice-span (EVS), translation accuracy and percentage of omissions were measured as dependent variables on which previous preparation had a significant effect.