We have examined university physics students’ use of models when explaining phenomena concerning the interaction between electromagnetic radiation and matter. A range of scientific models are available to explain the phenomena. The student sample is drawn from six universities in UK and Sweden. The sample students have difficulties in providing appropriate explanations for the phenomena. Few students use a single model consistently in their explanations of related phenomena. Students' use of models appears to be highly sensitive to the context in which the phenomena to be explained is presented to them.