Problems in e-commerce can lead to significant consumer harm (detriment), for example, when consumers have to pay more for a product than they reasonably expected (e.g. due to hidden costs); suffer from unfair contract terms; or receive products that do not conform to their reasonable expectations regarding quality or performance. In 2021, the OECD’s Committee on Consumer Policy implemented an online consumer survey in 13 countries to assess the magnitude of financial consumer harm arising from consumers’ most serious e-commerce problems. This paper summarises the survey results, highlighting, for example, that 50% of online consumers faced at least one problem in e-commerce in the year preceding the survey rollout and that the resulting harm is significant, reaching (after redress) up to 3.1% of the total e-commerce market size in some countries. The paper aims to enhance the evidence base for consumer policy and to help policy makers prioritise enforcement activities.