It has been assumed that mathematics testing indicates the development of mathematics concepts, but the linguistic demands of assessment have not been evaluated, especially for children with mild intellectual disabilities. 244 children (grades 2 – 5) were recruited from a larger reading intervention study. Using a multilevel longitudinal SEM model, baseline and post-intervention time points were examined for the contribution of item linguistic complexity, child language skills, and their potential interaction in predicting item level mathematics assessment performance. Item linguistic complexity was an important, stable, and negative predictor of mathematics achievement with children’s language skills significantly and positively predicting mathematics achievement. The interaction between item linguistic complexity and language skills was significant though not stable across time. Following intervention, children with higher language skills performed better on linguistically complex mathematics items. Mathematics achievement may be related to an interaction between children’s language skills and the linguistic demands of the tests themselves.