This cross-sectional study identifies key conceptual difficulties experienced by upper secondary school and pre-service chemistry teachers (N = 191) in the area of reaction rates. Students' ideas about reaction rates were elicited through a series of written tasks and individual interviews. In this paper, students' ideas related to reaction rate and its relationship with concentration or pressure are discussed. Evidence is presented to support the following claims. First, school students tended to use "macroscopic" modelling rather than using "particulate" and/or "mathematical" modelling. By contrast, undergraduates were more likely to provide explanations based upon theoretical models and entities within established chemical ideas. Nevertheless, second, they had conceptual difficulties in making transformation within and across different theoretical models. Finally, students did not generally use a scientifically acceptable concept of reaction rate across contexts. Although an acceptable concept may have been used in one context, incorrect ideas may, nonetheless, have been used in other contexts. However, undergraduates' responses were less affected by context. Several conceptual difficulties exhibited by school students persisted among undergraduates. Some possible implications for planning the curriculum and teaching are proposed in the light of the results. (Contains 3 tables and 3 figures.)