The purpose of this study was to compare the effects of a conceptual change approach over traditional instruction on tenth-grade students' conceptual achievement in understanding chemical equilibrium. The study was conducted in two classes of the same teacher with participation of a total of 44 tenth-grade students. In this study, a pre-test/post-test control group semi-experimental design pattern was used. During teaching the topic of chemical equilibrium concepts in the chemistry curriculum, a conceptual change approach was applied in the experimental group whereas traditional instruction was followed in the control group. Data were analysed with an independent samples t-test, and an analysis of covariance using the pre-test scores as the covariate. The results showed that the conceptual change approach was statistically more effective than traditional instruction in terms of students' conceptual understanding. After the six-week intervention, interviews were conducted with 18 selected students in the study. In the interviews a prevailing alternative conception was detected beside many alternative conceptions. When they were asked to compare equilibrium constants of two different reactions they mostly responded to mean that "the bigger the mol number of the products, the bigger the equilibrium constant" without considering what proportions the reactants were transformed into products in a reaction. Also, in this study a two-tier multiple choice test was developed related to chemical equilibrium to detect students' concepts and alternative concepts. (Contains 3 tables.)