Chemical equilibrium is one of the most important yet difficult topics to teach. The purpose of this study was to examine how two science teachers experienced the mediation of and how they mediated learning of this topic, chemical equilibrium, to high-achieving students. Data were collected using semi-structured interviews, lesson observations and stimulated recall interviews. The five topic-specific pedagogical content knowledge (TSPCK) components, namely, teacher knowledge of student prior knowledge, curricular saliency, what is difficult to understand, representations and conceptual teaching strategies, were used as conceptual tools to analyse the data. The findings revealed that the two science teachers struggled to mediate learning of chemical equilibrium concepts to high-achieving students and therefore the approaches used to teach them should not necessarily be any different from those used for average-ability students. Similarly, the teacher's role as a mediator of meaningful learning is equally important for high-achieving students. We thus recommend that there is a need for continuing professional development for science teachers focussing on the development of materials and strengthening of their TSPCK components.