The purpose of this study was to improve preservice chemistry teachers' understanding of surface tension by using guided-inquiry activities. Ten preservice chemistry teachers participated in this study. Data were collected with open-ended questions on the effect of molecular weight, branching, and temperature on surface tension before and after guided-inquiry. The results indicated that most of the participants had difficulties in understanding surface tension and could not provide scientific explanations before the guided-inquiry activities. However, after the guided-inquiry activities, most of the participants were able to provide true or partially true responses and also to present scientific justifications in terms of the effect of molecular weight and the effect of branching on surface tension. Moreover, the study revealed that guided-inquiry activities immensely helped participants to understand the effect of temperature on surface tension, especially when compared to the effect of the other two factors. The study found that guided-inquiry activities improved the understanding of the preservice chemistry teachers about surface tension. In light of the results of this study, implications are stated for preservice chemistry teacher education and chemistry education researchers.