Volunteer labor is generally analyzed as a homogeneous activity, implying that the marginal effects of tax changes and demographic shifts are equal across industries and forms of volunteering. Here the homogeneity assumption is tested by estimating and comparing volunteer labor supply functions in three sectors that rely on volunteer labor—health, education, and religious organizations. Differences in the marginal volunteer labor supply effects are associated with personal demographics, household composition, and tax status. These differences are significant statistically as well as for their policy implications. The effects on volunteering to each sector are predicted for changes in the age, education, and marital status distributions of the population, as well as for changes in income tax rates, itemization status, and income. © 2002 by the Association for Public Policy Analysis and Management.