In recent decades, research has shown the challenge posed to teachers by science education in informal contexts such as science museums or centres. In addition, there is consensus that in the case of pupils visiting a science centre, learning improves when the visit is connected to the classroom curriculum, so the teachers' involvement in preparing a visit is essential. In this work, we present research into primary and secondary school teachers' behaviour when preparing a school visit to the "Eureka Science Museum" in San Sebastián. Semistructured interviews were conducted for 158 teachers (87 from primary and 71 from secondary education). The results show that most teachers who visit this centre with their class group scarcely prepare for the visit, i.e. they do not have a clear idea of how to use the museum as an informal resource for learning about science.