The importance of research on misconceptions about chemical equilibrium is well recognized by educators, but in the past, researchers' interest has centered on student misconceptions and has neglected teacher misconceptions. Focusing on the effects of adding more reactants or products on chemical equilibrium, this article discusses the various misconceptions held by high school teachers. A misconception test was administered to two samples of chemistry teachers in Nanjing, China. Of the 109 teachers who participated in the test, only one understood that adding more CS[subscript 2] gas to the equilibrium system CS[subscript 2](g) + 4H[subscript 2](g) [rightwards harpoon over leftwards harpoon] CH[subscript 4](g) + 2H[subscript 2]S(g) at constant pressure and temperature can shift the equilibrium to the reactant or product side, depending upon the amount of CS[subscript 2] in the initial equilibrium system. Most of the teachers relied on Le Chatelier's principle and thus made erroneous predictions. The misconception test also revealed that those teachers who managed to compute equilibrium constants had a limited conceptual understanding of chemical equilibrium. Implications of these findings for teacher education and chemistry curriculum development are discussed.