This study investigates Swedish students’ ability to produce the discourse of the subject history, in a situation where they had to demonstrate historical knowledge in written explanations, and where both English and Swedish were involved. The students attend a Content and Language Integrated Learning (CLIL) programme at the upper secondary school level. The study highlights the roles of both languages in CLIL, thus including both English and the L1 Swedish. The research questions concern language choice, field-specific lexis and genre. These variables were related to the teacher’s assessment in terms of grading, in order to discover possible connections between linguistic choices on the part of the students and the degree of success in terms of grades. While the students had quite a lot of input in English during the history course, they mainly choose to write the exam in Swedish. The elements of English in the texts were primarily used for field-specific lexis. Texts with elements of English also tend to be slightly less successful than texts written in Swedish only. The overall achievements in this exam were quite low, even though the students used their strongest language. Few students display the significant linguistic resources needed in the production of successful historical explanations. Texts assessed with high grades represent content by means of linguistic choices that correspond to the typical patterns of the field, more than the texts with lower grades. The results confirm the importance of paying attention to both languages in CLIL education.