This study investigated the effect of using a non-traditional writing task and different audiences on 9th grade students’ conceptual understanding of a chemistry unit on mixture. 524 students instructed by 3 chemistry teachers in 2 public high schools participated in this study. Upon completion of the mixture unit, 181 students engaged in traditional writing activities, 121 students wrote a letter to a younger audience, 92 students wrote a letter to their peers, and 130 students wrote a letter to their teacher. Mixture achievement test was administered as a pre- and posttest to all the groups to assess students’ understanding of mixture concepts. Semi-structured interviews were conducted with 24 students from the non-traditional writing group at the end of the instruction to understand students’ ideas about non-traditional writing task. Posttest analyses indicated that non-traditional writing group outperformed the traditional writing group, and the groups that wrote to peers and younger students performed better than those that wrote to the teacher, when the effects of the pretest scores were controlled. The results also showed that students’ performance on the writing task significantly differed with respect to the audience. Moreover, interview results revealed that non-traditional writing tasks were very helpful in students’ understanding of the mixture concepts.