A critical step in problem-based learning (PBL) units occurs when groups present their solution to the central problem. This is challenging for middle school students because it involves the creation of an evidence-based argument (Krajcik et al., "Journal of the Learning Sciences" 7:313-350, 1998). Using a mixed method design, this study investigated (a) the impact of computer-based argumentation scaffolds on middle school students' argumentation ability, and (b) what middle school students used for support and why during a PBL unit. Data sources included persuasive presentation rating scores, argument evaluation ability test, videotaped class sessions, and prompted interviews. Results included a significant impact on average-achieving students' argument evaluation ability, and use of the scaffolds by the small groups to plan their research and keep organized.