The influence of language or subject-specific language proficiency on mathematics achievement could be supported by numerous empirical results. The socioeconomic status of students also proved to be influential in this regard. This is related not only to mathematical, but also language achievement. Based on the data of 438 elementary school students, it was therefore examined to what extent the influence of the socioeconomic status on the mathematics achievement is mediated by the subject-specific language proficiency. The educational qualifications of the parents and the number of books in the household (book index) were used as indicators for the socioeconomic status, the influences of whose were also considered in comparison. Considering the intelligence quotient and the migration background, the results of the latent variable path models, indicate a partial mediation by the subject-specific language proficiency. After including the mediator in the model, the previously analysed direct effect remained significant, but was considerably decreased. This applies regardless of the indicators chosen. The findings of the multilevel analysis also appear to be significant: It was found that the influence of family background characteristics and subject-related language proficiency on mathematics achievement varies between different schools.