This research provides new evidence about the relationship between online and in-store shopping. This article uses data for England from the UK National Travel Survey (NTS) which covers a full week and also provides information about grocery shopping practices (online and in-store). We examine the effects of online purchases on the grocery shopping practices of working couples and their related CO2 emissions. Our analysis reveals a substitution effect between in-store trips and online shopping combined with home delivery and a 37% reduction in household CO2 emissions for grocery shopping. It appears that buying groceries online combined with a home delivery system makes it possible to significantly cut emissions for grocery shopping trips, or at least to offset the effects of in-store trips by the households with the highest emissions. No rebound effect is observed for other trip motives, i.e. it is not because people make fewer trips for in-store grocery shopping that they make more trips for leisure, personal or other motives.