Recent years have witnessed a growing interest in the relationship between academic language registers and school success in the German-speaking education system. However, we still know very little about the actual effects that academic language has on the academic performance of students, for instance, in how far the extent to which academic language is used in subject tasks actually makes these tasks more difficult. It is therefore highly vital that any operationalization of difficulty-inducing linguistic features of tasks is made on solid theoretical and empirical grounds. The purpose of this article is thus to present the linguistic foundation used in an interdisciplinary empirical study in which 1.346 7th and 8th graders solved a set of subject-oriented tasks from Maths, Physics, German, PE and Music, while the degree of linguistic demands in the tasks was systematically varied. First, the theoretical and empirical research on linguistic difficulty from a range of research discourses is discussed. The findings are merged into a model of linguistic demands. Its operationalization is then illustrated in three linguistically varied versions of the subject-specific tasks. Finally, an outlook on preliminary results of the empirical study is given, which indicate that the categories used in the model actually do produce differences in subject-task difficulty, even though there are a number of effects that need further investigation.