This study aimed to investigate the effect of the peer instruction method on students' conceptual understanding for teaching about solutions at the high-school level and to compare peer instruction with the existing method. Furthermore, the effect of the peer instruction method on students' attitudes toward chemistry class were also examined. A mixed-method design was employed in this study. Participants consisted of 59 eleventh graders from two different classes at Artvin Iskebe Anatolian High-school during the 2016-2017 academic year. The study group was selected using the convenience sampling method and one of the classes was randomly assigned to be the experimental group and the other to be the control group. The peer instruction method was used in the experimental group and the existing method was used in the control group. The application stage lasted for four hours per week over the course of five weeks. The data for the study were collected using the Solutions Concept Test (SCT), Attitude Scale toward Chemistry Course (ASTCC), concept questions, semi-structured interviews, and observations. Independent group t-tests were used to analyze the data obtained from the SCT and ASTCC. Data obtained from semi-structured interviews and observations were analyzed descriptively. The average SCT post-test score of the students in the experimental group was higher than that of the students in the control group. The results of the independent t-test analysis revealed that this difference was statistically significant. According to these results, it can be argued that the peer instruction method is more effective than the existing method in promoting understanding of the concepts of chemical solutions, and the qualitative data obtained in the interviews also supported these findings. There were no statistically significant differences between the experimental group and control group in terms of students' attitudes. Despite this, it can be asserted that the students in the experimental group adapted to the peer instruction method throughout the application period and that they enjoyed participating in the lessons taught in this way. Based on in-class observations, it can be argued that the students' in-class discussion skills improved. Overall, this study suggests that the peer instruction method is appropriate for promoting conceptual learning and correcting misconceptions.