A study investigated the relative effects of ownership opportunities and instructional support on high school students' engagement with assigned writing topics. Subjects, 66 eleventh graders enrolled in five United States history classes, experienced three instructional conditions, writing on one of three topics in each one. The instructional conditions were: ownership opportunities/no instructional support; instructional support/no ownership opportunities; and ownership opportunities/instructional support. Data were also collected on students' self-efficacy, topic-knowledge organization, task-related persistence, behavior, essay characteristics, and students' responses to the instructional/writing episodes. While the results emerging from this study were highly complex, the major finding indicated the importance of instructional support in explaining the students' writing-task engagement. Students were more persistent, more reflective, expressed less self-doubt and greater interest when instructional support was high. (Two figures and four tables of data are included. Fifteen references are attached. An appendix includes the self-efficacy and student response questionnaires.) (SR)