This paper reports an experimental study on Form Four (16 year olds) students' conceptions about two acid-base models (Arrhenius & Brønsted-Lowry). An intact class was divided into two groups that were taught different acid-base models. One teacher taught one of the groups the Brønsted-Lowry model while another teacher taught the Arrhenius model. The acid-base diagnostic instrument (ABDI) comprising 12 items was administered to 34 Form Four students. Two items from the ABDI were analyzed with one selected sub-concept. The research findings suggest that the Brønsted-Lowry group of students believed that water can be an amphoteric substance suggesting that the students exposed to the Brønsted-Lowry model had better comprehension of the amphoteric nature of water. Implications are then made with recommendation to include the Brønsted-Lowry model in the Form Four Malaysian acid-base curriculum.