Relevant market definition remains a key element in the economic analysis of competition in the gasoline market. However, this is particularly difficult to undertake when competition is local and market power is geographically constrained, as they are in the case of the gasoline market. We analyse how the hypothetical monopolist or Small but Significant Non-Transitory Increase in Prices test performs when defining isochrones based solely on price information and the distance between competitors. We conclude that geographic information systems can be successfully employed in the precise definition of relevant geographic markets in the gasoline retail sector. Their application to the Spanish gasoline market indicates that the relevant geographic market is delineated by a 5- to 6-min travel-time isochrone around each station. Localized market power needs to be taken into account when analysing the adverse effects of mergers and entry regulations on gasoline retailing. To drive competition in these local circumstances, markets need to be delineated on the basis of sufficiently small isochrones since only close rivals seem to compete effectively with each other.