Language proficiency has proved to be relevant for mathematical learning (e. g., Bochnik & Ufer, 2016; Daroczy et al., 2015; Prediger et al., 2019). A diagnosis of language proficiency in mathematics can form the basis for promoting individual and subject-specific language measures (Häsel-Weide & Prediger, 2017). The diagnostic instrument SAMT enables the diagnosis of language proficiency in mathematics in primary-school children. This article concerns the validation of the diagnostic instrument and with that the representation of its psychometric quality. The validation was embedded in the research and evaluation project Eva-Prim, funded by the BMBF. The sample on which the validation study is based comprises 384 students in the 3rd and 4th grades. The results provide evidence for satisfactory reliability, factorial validity, and criterion validity as well as sensitivity to change. Further, we could substantiate measurement invariance between the sexes (see Merkert, 2022).