High annual turnover among volunteers heightens policy and practitioner concern about effective retention strategies. Volunteer commitment is a complex interaction between the antecedents to volunteering, including a volunteer’s personal characteristics and motivations, and situational factors of the volunteer experience such as organizational practices that encourage sustained volunteerism. Using the Penner (2002) volunteer process model to illustrate this interactive approach, I estimate future volunteering intentions among a distinct group: Members of occupational associations. Data come from a large international pool of professional and occupational society members, and analyzed using GZLM and multinomial logistic regression. The findings suggest that the strongest influences on sustained commitment come from situational factors related to the volunteer experience rather than prior social conditioning. The findings support theory building on volunteer motivations generally and are also useful in building an understanding of voluntary membership behavior in professional associations, where research is still in the early stages.