The paper examines consumers' perceptions of grocery retail brand extension to a non-traditional offer, namely petrol stations. Retail brand extension literature is limited, particularly as regards new products and services domains in which grocery retailers are increasingly directly operating through their brands. Additionally, the role of product knowledge as a moderator of the attitude-purchase intention relationship requires further insights. To these aims, we propose a model in which product knowledge (PK) is tested as a moderator of the relationship between attitude towards the extension (ATEX) and intention to buy the extended product (INTB). At the same time, we tested the role of ATEX as mediator of the impact of a series of antecedents-conceptual fit (FIT), national brand preference (NBP), risk (R) and resources and competences (R&C). A survey on a sample of grocery retail customers was performed administering a structured questionnaire. Then, data were processed applying Structural Equation Modelling (SEM). The conceptual model achieves good predictive validity and all the proposed hypotheses are supported. Some preliminary theoretical and managerial implications are derived.