The purpose of the present study is to examine the language characteristics of a few states' large-scale assessments of mathematics and science and investigate whether the language demands of the items are associated with the degree of differential item functioning (DIF) for English language learner (ELL) students. A total of 542 items from 11 assessments at Grades 4, 5, 7, and 8 from three states were rated for the linguistic complexity based on a developed linguistic coding scheme. The linguistic ratings were compared to each item's DIF statistics. The results yielded a stronger association between the linguistic rating and DIF statistics for ELL students in the "relatively easy" items than in the "not easy" items. Particularly, general academic vocabulary and the amount of language in an item were found to have the strongest association with the degrees of DIF, particularly for ELL students with low English language proficiency. Furthermore, the items were grouped into four bundles to closely look at the relationship between the varying degrees of language demands and ELL students' performance. Differential bundling functioning (DBF) results indicated that the exhibited DBF was more substantial as the language demands increased. By disentangling linguistic difficulty from content difficulty, the results of the study provide strong evidence of the impact of linguistic complexity on ELL students' performance on tests. The study discusses the implications for the validation of the tests and instructions for ELL students. (Contains 10 tables, 2 figures and 1 footnote.)