The aims of the current project were to: (1) describe average change in the use of academic words across the school year in writing and (2) examine potential predictors of growth in academic word use in response to morphology-focused supplemental instruction. Investigators utilized written expository responses of 824 students in 5th grade before and after a 20 week supplemental language instruction that focused on morphological knowledge in academic words. Students received supplemental Word Knowledge Instruction (n = 515) or business-as-usual (n = 307). The sample included students who were English Learners (ELs), previous ELs with fluent English proficiency (FEP), never ELs, and students with language-based learning disabilities (LLD). Small but significant increases in academic word use in written responses were evidenced from fall to spring. Results from a two-level hierarchical linear model with students nested within teachers indicated that a supplemental instructional approach predicted change in academic word use from fall to spring for students with differential effects by EL status. The morphology-focused supplemental instruction was related to larger gains in academic word use in writing than business-as-usual for students with LLD and FEP students. ELs with limited English proficiency showed similar outcomes in both conditions.