Eco-labels are increasingly important to certify food that is produced under sustainable conditions. In this paper variables are analysed that are important for consumers’ purchase decisions of eco-labelled food exemplified by the case of Fairtrade products. The focus lies on the distinction between people’s subjective (perceived) knowledge and objective knowledge (actual) about the Fairtrade label. The empirical data for the study was gathered by a survey, which involved a quota sampling of 203 people in Berlin. The results justify the distinction between subjective and objective knowledge. Subjective knowledge proved to be a strong predictor for both attitude and consumption towards the label, whereas objective knowledge did not show a significant influence. Further, attitude in general was confirmed to be a predictor for the consumption of Fairtrade products.