Purpose ‐ The purpose of this paper is to apply a structured approach to understand the importance of personal ecological norms in purchasing organic food. The norm-activation-model by Schwartz is used to predict self-reported and observed purchase behaviour of organic milk. Design/methodology/approach ‐ The paper reports the results of a field study with 63 customers of a German supermarket. A combination of covert observation and in-store interviews was applied to obtain reliable data on actual shopping behaviour and its predictors. Findings ‐ The results show that the self-reported and the observed purchase of organic milk is predicted by personal ecological norms, social norms, and perceived behavioural control. Personal norms are activated by awareness of need, awareness of consequences, perceived behavioural control, and social norms. People with strong personal norms use "organic production", the "EU-BIO-Label" and "ingredients" as additional criteria during their decision process. For people with strong ecological norms the price difference between organic and conventional milk, the lack of knowledge about organic milk, and convenience are less important constraints. Finally, people with strong personal norms react more sensitively to proposed norm-centred interventions. Practical implications ‐ The study offers insight into the processes of motivating behaviour and can therefore be used to design intervention strategies. Suggestions are developed in the closing part of the paper. Originality/value ‐ The study applies for the first time the norm-activation-model to the domain of purchasing organic milk and underlines the importance of normative influences for this decision.