There are many studies in chemistry investigating students’ difficulties in understanding chemical bonding, particle nature of matter and others, but relatively few on acid-base chemistry. One approach for reducing students’ learning difficulties uses model-based science teaching, which involves models and mental models. This thesis study investigates students’ mental models in acid-base chemistry concepts to give insights into Malaysian secondary students’ thinking in acid-base chemistry. In addition, teachers’ mental models and the curricular models were also examined in order to explore the degree of alignment between the three models. At Forms 2, 4, and 6 levels of Malaysian schooling eight secondary school students and two teachers were interviewed at each level in an effort to examine their mental models using the Interview-About-Concepts and Interview- About-Instances data gathering methods. In addition, Forms 2, 4 and 6 curricular models (i.e., curriculum documents) were examined to obtain insights into the curricular models. The area under investigation for this thesis study involves six selected acid-base chemistry concepts and their links to four acid-base models. The six selected acid-base chemistry concepts are Macroscopic Properties, Neutralisation, Acid-Strength, Acid-Base Equilibrium, Buffers, and Acid-Base Electron Pair Bonding while the four acid-base models are the Phenomenological, Arrhenius, Brønsted-Lowry, and Lewis models. To determine the nature of students’ mental models, attributes of these models were identified and gathered from students’ expressed models, that is their responses to probe questions about the selected acid-base concepts and compared with the attributes of each scientific acid-base models. This comparison provided evidence of students’ use/non-use of the attributes of the appropriate acid-base models to explain six selected acid-base chemistry concepts. Next, a mental model framework was developed and used to classify students’ attributes into Stage 1, Stage 2, and Stage 3 mental models. The Stage 1 mental model was developed based on the Macroscopic Properties acid-base chemistry concept to indicate students’ use or non–use of the Phenomenological model. The Stage 2 mental model was developed to determine students’ use or non-use of the Arrhenius model to explain the Neutralisation and Acid-Strength concepts. The Stage 3 mental model comprising the Acid-Base Equilibrium, Buffers, and Acid-Base Electron Pair Bonding concepts were investigated to identify students’ use or non-use of the Brønsted-Lowry and the Lewis model. Also, under investigation was a comparison of students’ mental models with teachers’ mental models and the curricular model. At Form 6 schooling level the students’ mental models demonstrated complete dissonance with the teachers’ mental models and the curricular models. The causes for this dissonance may be the lack of specificity in the Malaysian curriculum, students’ limited cognitive ability in terms of age-approriate concepts, and insufficient teachers’ pedagogical knowledge. From the findings of this thesis study, it is recommended that the Lewis acid-base model, be omitted from the Form 6 Malaysian curriculum because students’ were not able to understand Acid-Base Electron Pair Bonding chemistry concept. Also, for other acid-base chemistry concepts, Malaysian teachers are encouraged to use student-centred teaching methods utilizing acid-base models to help improve their students’ understanding.