Nine 12th grade students, attending the science section of the upper secondary school in Cyprus, were individually interviewed about their understanding of the changes associated with the process of oxidation/combustion. Each student was instructed to firstly predict what would happen when heating a piece of copper wire or a piece of magnesium strip, and, second, to heat each of them with a Bunsen burner flame. Following each experiment, he/she was then asked to describe and explain its outcome(s) in terms of the macroscopic and/or microscopic changes associated with it. Questions were also raised concerning the conservation of mass, the reduction of copper oxide, and the energy transformations due to bond breaking and reforming. The results of the analysis showed that the students had limited understanding of chemical reactions, while their answers indicated that they accumulated extended declarative knowledge, which had not been assimilated. Taking into consideration that students had repeatedly studied "redox" reactions, both from an "oxygen-based" and an "electron-based" definition, the results cast doubt on the appropriateness of the existing curricula and/or the teaching practices, and several recommendations are put forward. These recommendations stem from the point of view of learning as a construction process and address the need to encourage teacher-driven action research, and employ teaching that takes into consideration the complex nature of chemistry concepts, and the way chemists represent them. (Contains 1 table.)