A growing body of literature in collaborative natural resource management has identified factors associated with group processes, outputs and outcomes. Scholars have also begun to identify variables that affect whether, why and how volunteers choose to participate in collaborative efforts. To date, however, these variables have been analysed largely in isolation. This study examines variables from the Theory of Planned Behaviour and volunteerism literature. Analysis of data from members of 11 watershed partnerships in Ohio, USA, indicates significant links from attitudes, subjective norms (how ‘important others’ expect an individual to behave), and personal requests to active participation in group activities.