This study measured the effect of mode of discourse and developmental factors on composition length, syntactic complexity, and sentence-production error rate in the writing of secondary school students. The study also included a descriptive analysis of syntactic and logical patterns found in the sentence production errors. The 297 students whose writing samples provided the data for this study were enrolled in grades 7, 9, and 11. The students were divided into low and high within-grade developmental groups. Each student wrote two compositions, one in the descriptive mode and one in the persuasive mode.