Results from national and state writing assessments suggest teachers struggle to meet the writing needs of students with disabilities as they score significantly lower than the overall population. With the implementation of Common Core State Standards in 2010, text-based writing became increasingly important for academic success both in the classroom and on standardized assessments. For students with disabilities, the complexity of text-based writing, where students are expected to state a clear purpose and use multiple text sources to support their analyses, is increasingly difficult, suggesting further targeted instruction is needed. While evidence suggests mentor text instruction, or the study of models, is an effective form of writing instruction for adolescents, further research is needed on its impact on the writing of middle school students with disabilities. Thus, this study used a multiple-probes across dyads design to investigate the achievement effects of small group mentor text instruction, using the "Discovering Mentors" intervention, on the text-based writing quality of middle school students with disabilities identified as struggling writers. Results suggest there is a functional relationship between "Discovering Mentors" and increases in the text-based writing quality of participants. Additional analyses show statistically significant effects on two of the three factors determining writing quality and participants' holistic writing scores. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]