This study presents an analysis of alternative conceptions (ACs) on acid--base chemistry harbored by grade 9 students in Singapore. The ACs were obtained by the development and validation of a 4-tier diagnostic instrument. It is among the very few studies in the science education literature that have focused on examining results based also on measures used in the educational psychology literature. The results indicate that the students harbor a range of ACs of varying strengths in relation to the properties of acids and bases, strengths of acids and bases, pH, neutralization, indicators, and sub-microscopic views of acids and bases. The 25-item instrument uncovered 30 ACs. A novel insight from this study is that when students are presented with a test item with all incorrect answer and reason responses but with a blank space for them to fill up their own answer and reason if they disagree with any of these responses, hardly any chose to do so. ACs were also identified from this question. Overall, the results reiterate the utility of the 4-tier format for identifying ACs and getting some indication of their strengths.