The goal of this comparative study was to determine the knowledge that French and Turkish upper secondary-school students (grades 11 and 12) acquire on the concept of acid-base reactions. Following an examination of the relevant curricula and textbooks in the two countries, 528 students answered six written questions about the acid-base concept. Student ideas on the concept of acid-base reactions and how these opinions evolved over time were examined. The study shows that many students have difficulty understanding the concept of acid-base reactions even after being taught about it. Students' conceptions were classified into three mental models. This article discusses implications for teaching the concept. (Contains 1 table.)