We report in this paper on a study of chemistry teachers’ perceptions of their teaching in upper secondary schools in Sweden, regarding models of acids and bases, especially the Brønsted and the Arrhenius model. A questionnaire consisting of a Likert-type scale was developed, which focused on teachers’ knowledge of different models, knowledge of students’ difficulties, and use of textbooks in their teaching of acids and bases. The questionnaire was sent to 441 upper secondary schools in Sweden and a total of 281 teachers answered it; through a cluster analysis they were divided into three subgroups. From each subgroup, two teachers were interviewed in order to enrich the data. The results indicate that Swedish upper secondary chemistry teachers, on the whole, prefer to use the Brønsted model of acids and bases, and think that the Brønsted model is clear for students. However, in cluster one (47% of the teachers), teachers’ knowledge of how the Brønsted model differs from the Arrhenius model was limited and varied. Teachers in clusters two (38%) and three (15%) were better aware of the differences between the Brønsted and older models, but only teachers in cluster three did explain the history of the development of knowledge about acids and bases in their teaching. The teachers in cluster two (like the teachers in cluster one) relied more on the content in the textbooks. There were, however, no differences between the three clusters in terms of knowledge of student difficulties. Implications for chemistry teaching and education, and for further research, are discussed.