Abstract The qualitative research project EKoN-E (Entscheidungsprozesse von Kindern im Kontext einer Nachhaltigen Entwicklung mit dem Fokus Ernährung [Decision-making processes of children in the context of sustainable development with a focus on diet]) aims to investigate how school pupils (age 11–12 years) carry out decision-making processes and the role that their knowledge and values play in this. Part 1 of the article focuses on the role of knowledge. Using a learning sequence, school pupils (N = 97) were prepared to make decisions through the development of knowledge, among other measures. Afterwards, the course of their decision- making processes was recorded through individual interviews (n = 27) using the thinking aloud method. Qualitative analysis of the content was then performed using Mayring′s method. The results show that the level of knowledge generally does not determine how the decision-making process plays out. However, ″expert knowledge″ about certain individual aspects of sustainable development appears to be a prerequisite for children using knowledge of the relevant aspects in their decision-making processes. Therefore, it follows that lessons should support both the development of knowledge and the incorporation of knowledge into decision- making processes in a targeted manner.