This study examined whether the application of conceptual change texts are effective before or after the instruction on 10th grade students' conceptual understanding and alternative conceptions about acids and bases. The study was conducted with 76 10th grade students from three classes of a chemistry course taught by the same teacher. One of the classes was randomly assigned to the control group (N=25), who were taught with traditional teaching methods, and another class was randomly assigned as experimental group 1 (N=26), who studied the conceptual change texts before the traditional teaching. The other class was randomly assigned as experimental group 2 (N=25), and studied the conceptual change texts after receiving traditional teaching. The data was collected by the Concept Achievement Test, administered as pre-test and post-test. The analysis of covariance (ANCOVA) is used to analyze the data. The analysis revealed that the differences between the results in both the experimental groups and in the control group were statistically significant. (Contains 5 tables.)