In recent years, regulators in various parts of the world have capped interchange fees on debit and credit cards. The justification for the caps rests to a large extent on the argument that these cards have, for certain merchants, become must-take cards rather than ‘wanna-take cards’. That is, there are merchants who accept payment cards not because they bring net convenience benefits but out of fear of losing profitable business to card-accepting competitors. This paper presents an original approach that allows to quantify, for the first time, the relative importance of the two motivations. We find, for the case of France in 2008, that the must-take phenomenon effectively exists, but that it applies to only 5.8-19.8 per cent of the card-accepting merchants and to a mere 3.9-13.5 per cent of all retailers.