This study tested whether writing skills, knowledge, motivation, and strategic behaviors (within the context of robust writing instruction) each made a statistically unique contribution to predicting fifth-grade students' (123 girls, 104 boys) composition quality and length on a persuasive writing task involving source material, after variance due to other predictors and control variables (reading comprehension, gender, class, and school effects) were controlled. With one exception, writing skills, knowledge, motivation, and strategic behaviors each accounted for statistically unique variance in predicting compositional quality. The exception involved writing knowledge, which did not make a unique contribution in the fall but did in the spring, when a topic knowledge measure was added. In addition, writing motivation, and strategic behaviors accounted for unique variance in composition length in the fall, and writing knowledge did so in the spring.