This study explored Swedish preschool children's knowledge about the environmental impact of various transport modes, and investigated whether or not eco-certification has any role to play in relation to this knowledge. Additionally, this study examined children's perceived sources of knowledge. Using illustrations and semi-structured questions, 53 children, aged five to six years, from six eco-certified and six non-eco-certified preschools were interviewed. Qualitative and quantitative data were analysed using content analysis and Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA), respectively. Findings revealed that most of the children had acquired some knowledge about the environmental impact of various transport modes, although some children were not familiar with the word 'environment'. Although the complexity of children's justifications for the environmental impact of different modes of transport tended to be higher at eco-certified preschools compared to non-eco-certified preschools, no statistically significant differences were found. Parents were reported to be a major source of knowledge.