This study aims to determine prospective science student teachers' alternative conceptions of the chemical equilibrium concept. A 13-item pencil and paper, two-tier multiple choice diagnostic instrument, the Test to Identify Students' Alternative Conceptions (TISAC), was developed and administered to 90 second-semester science student teachers enrolled in CHEM 102 Chemistry II course in spring 2006, after they received fourteen 50-minute regular course instruction concerning the equilibrium. The content validity of the test was established by the panel consisting of lecturers. The Spearman-Brown reliability for the test was 0.71. Analysis of the results collected with the TISAC show that students did not acquire a satisfactory understanding of the chemical equilibrium concept. For the first tier of the test items, the range of correct answer was 48.8% to 78.8%. When both tiers were combined, the correct response was reduced to a range of 22.2% to 48.8%. In this study, seventeen alternative conceptions were also identified through analysis of the TISAC. These conceptions were grouped under the headings of the application of Le Chatelier's principle, reliability of the equilibrium constant, heterogeneous equilibrium, and the effect of a catalyst. (Contains 4 tables.)