This paper reports on a longitudinal study on how science student teachers' reasoning about a complex environmental issue developed through a teacher education programme in mathematics and science for grades 1-7 (ages 7-13). Of special interest was to follow the ways in which student teachers used scientific knowledge in their reasoning. The issue was taken from a newspaper article discussing the use of surplus heat from a crematorium for district heating. A group of 14 student teachers were followed through two and a half years of a teacher education programme, during which they were interviewed three times. The results showed that the student teachers drew upon scientific knowledge only to a limited degree when they were asked to clarify the situation in the article, quickly taking standpoints based upon emotional arguments. Several student teachers experienced a contradiction between science and their emotions. It was also shown that there was a conceptual problem, which could be important for how the student teachers understood the issue, and thereby influenced their decision-making.