Health interventions commonly have adverse effects. Addressing these could significantly improve health outcomes. This paper addresses an adverse effect common in the promotion of health behaviours: exacerbation of health inequalities between low- and high-socioeconomic groups. Health behaviours – particularly, physical activity - are positioned within the context of social inequality and the inequitable spatial distribution of resources. Area-based health policy that targets deprived areas is assessed for its capacity to promote health behaviours without exacerbating inequality. Data are derived from a 16-month ethnography in a deprived English neighbourhood that was the target of area-based intervention that prioritised the promotion of physical activity. Findings provide evidence of adverse intervention effects that further disadvantaged the low-socioeconomic population. Analysis demonstrates how this was ultimately the outcome of localised policy drifting away from initial commitments to equitable service access. These findings increase understanding of the processes through which adverse intervention effects arise and how they can be mitigated.