A switch from single-to multichannel grocery shopping can affect consumers' purchase allocations. This study investigates whether and to what extent a consumer's decision to start buying groceries online at a particular chain can lead to expansion of the share in grocery spending allocated to that chain. To obtain a better insight into the underlying mechanisms and to provide more accurate and refined managerial guidelines, we further investigate the moderating effects of a comprehensive set of factors that may stimulate or withhold these expansion effects. To analyze these effects, we use a large and representative U.K. household panel data set, composed of multichannel grocery shoppers and a matched group of single-channel shoppers, for a 2.5-year period across all multichannel grocery chains. The empirical analysis confirms that multichannel grocery shoppers expand the share of wallet allocated to the online-visited chain, and that the extent of this expansion depends on chain as well as consumer characteristics. Expansion effects appear to be more profound for chains that adopt an integrated multichannel strategy for product prices and category assortments. In addition, customers who face more stringent time constraints, live farther away from the chain's offline store, and/or purchase fewer competitive private-label or hard discounter products, are more likely to increase their share of grocery spending at the online-visited chain.