A case study is presented describing the work of three pupils in the upper level of compulsory school. The pupils were learning about the intensified greenhouse effect and the depletion of the ozone layer. In their work, the need for certain domain-specific knowledge becomes apparent; for example, understanding such concepts as photosynthesis, radiation and catalyst. The results show that the pupils experienced difficulty in distinguishing between the different meanings attached to individual concepts in their theoretical and practical contexts. They are often unable to identify the meaning relevant to the specific environmental problem that they are working on. The pupils' difficulties are seen here as a problem of differentiation between concepts denoted by the same term as well as between different aspects of the same concept, a view that is then related to the question of appropriate conditions for conceptual change.