The current study explores the contributions of written vocabulary and syntax to informational writing quality in Grades 5 through 8. Various aspects of language skills (receptive to written, oral register to academic register) have differing relationships with narrative and informational writing achievement across development. In this study, we explore one relatively understudied aspect of language (written language in the oral register) during a key transition period. Specifically, we find support for the role of vocabulary and syntax skills in informational writing quality in early and later middle school; that the role may differ according to students' skill level; and we explore the role of written vocabulary and syntax in the gender gap between girls' and boys' informational writing quality.