This article discusses challenges involved in contrastive discourse analysis that emerged while carrying out a follow-up study into a Content and Language Integrated Learning (CLIL) program in Spain. Reversing the focus on English of much contrastive rhetoric work, the study investigates the effect of second-language-English on first-language-Spanish writing. The motivation for this focus and the choice of tools from Systemic Functional Linguistics (SFL) for genre and clause analysis are discussed. Reflecting on the difficulties involved in contrastive discourse analysis, in particular the challenges of comparing texts, it is suggested that contrastive work benefits from a more differentiating analytical method and a more dynamic conception of language. The implications of an influence from English are also considered, with the theses of hybridity and of homogeneity contributing to indicate a role for language awareness work in schools. (Contains 3 tables and 2 notes.)