Environmental third-sector organisations (EOs) aim to bring about social change by influencing not only policy making but also citizens’ attitudes and behaviours directly. While there is some evidence that direct involvement in environmental initiatives can encourage carbon-reducing behaviours, it is often believed that less direct forms of engagement are less effective in supporting behaviour change. This article provides a first direct comparison of associations between direct and indirect involvement with EOs and carbon-reducing behaviours, based on a representative UK survey. Results show that while direct involvement is more strongly associated with carbon-reducing behaviours than indirect involvement, both associations remain significant in the combined model. However, this only applies to more affluent respondents and not to those who, due to social disadvantage, do not participate in high-carbon activities. This suggests that EOs may further behaviour change through broader outreach work, especially among better-situated groups.