Studies of commons management make it clear that collective action for resource management is a highly complex process in which the impact of different conditions often will vary according to physical and socio-economic contexts. This paper attempts to contribute to the understanding of this process by exploring the causal mechanism of collective action through an examination of the intervening variables that connect contextual and policy factors with resource management outcomes in an indirect way. Using four hypothetical causal variables – a) degree of resource dependence; b) predictability of benefit flows; c) possibility of sanctions application; and d) possibility of trust building – and relying on the institutionalist framework,a comparative institutional analysis is applied to the community-managed rural water supply systems of two Senegalese villages. The analysis demonstrates that collective action is possible even when some of the facilitating conditions normally associated with successful commons management – such as resource scarcity and small/homogeneous user groups – are missing. It thus confirms that intervening variables are important for understanding the broader process of institutional change for sustainable resource management, and consequently to the crafting of more suitable policy interventions.