Previous studies have suggested that voluntary reform of the delivery of primary care services is more likely to occur in affluent areas. Health system reforms that include voluntary participation of GPs may therefore lead to a two-tier service in terms of access to and utilisation of medical services. New primary care organisations in Scotland (local health care co-operatives) were introduced in 1999. These are groups of general practices and membership was voluntary. The aim of this study is to examine whether the voluntary nature of membership was likely to exacerbate or reduce inequalities in the provision of primary care services. Logistic regression analysis was used to identify differences in population, practice, and GP characteristics between general practices that have joined a co-operative and those that have not. The results indicated that practices located in deprived areas and covering populations with high levels of morbidity were more likely to join a co-operative. High workload decreased the probability of membership. General practices that found it difficult to obtain access to local authority residential care homes were more likely to join a co-operative. The number of fee claims for minor surgery sessions per whole-time equivalent GP increased the probability of membership. There is therefore some evidence indicating that general practices located in areas of high need are more likely to join a co-operative. This suggests that voluntary participation in these new primary care organisations may reduce rather than exacerbate inequalities in the provision of primary care.