One of the complex units of chemistry education is acid-base equilibria, and learners have difficulty constructing cognitive structures on the subject. The aim of this study is to examine the conceptions of pre-service chemistry teachers with regard to weak acid-base equilibria using a diagnostic test. A qualitative research method was employed as the research design of the study. There were 57 pre-service chemistry teachers who were both graduates of a chemistry department in a faculty of science and participants of postgraduate teacher training programs in the study. The diagnostic assessment instrument was employed as a data collection instrument. The data were subjected to content analysis and coding separated them into groups. Pre-service teachers' conceptions were defined via these groups. In the study, two groups were determined: sound understanding and rote learning. Two lower levels at the sound understanding level and four lower levels at the rote learning level were also determined. These categories revealed the misconceptions of pre-service chemistry teachers with regard to weak acid-base equilibria. It was observed that many pre-service teachers had immature cognitive structures in respect to weak acid-base equilibria.