Understanding buying behaviour for sustainable products is important for food producers and distributors. This study aims to examine the effect of knowledge of the product, interest in sustainability, and characteristics of consumers on the frequency of buying sustainably produced butter using the Stimulus-Organism-Response (SOR) model. In this study, two different types of butter were used, one produced by a well-known brand and the other by a less--known dairy. Data were collected through a structured online survey of 825 respondents from Poland. The research found that knowledge about the product and the sustainability-related product attributes does not positively influence the frequency of purchasing sustainable butter. The results indicate that consumers living in smaller towns, having higher incomes and smaller households, are more frequent buyers of sustainably produced butter.