Voluntary measures are an increasing part of the environmental policy portfolio in most of the world. The economics literature on voluntary measures has found mixed evidence of their effect on environmental outcomes, controlling for the behavior of those who are not affiliated with the measure (non-partners) and trends leading up to the policy initiation. Traditionally, a voluntary measure would be labeled a success if the measure's partners statistically improved their environmental outcome compared to non-partners, once the measure was initiated. However, that type of evaluation assumes that voluntary measures provide the treatment exclusively to partners. In practice, for many reasons, the treatment provided to partners may also spillover to affect the behavior of non-partners. In voluntary programs, information on the reuse of an input can spread from partner firms to non-partner firms. If the researcher does not account for these treatment spillovers in the evaluation, it is possible for successful measures to be found unsuccessful.It is argued below that an alternative interpretation of a successful voluntary measure is needed, especially for those that involve treatment spillovers. Conditions under which a voluntary measure with treatment spillovers would be considered successful are discussed and then tested using the Coal Combustion Products Partnership (C2P2). The purpose of this voluntary program in the U.S. is to increase the reuse, as opposed to disposal, of coal combustion products (CCP). A difference-in-difference estimator finds that C2P2 partners are not statistically different from non-partners in their reuse rates, though the total reuse of coal combustion products has statistically increased over time. Further, non-partners located in states with more C2P2 partners statistically increase their reuse rate more than those in states with few C2P2 partners. The evidence is consistent with conditions needed for a program to be successful in reducing overall the disposal of CCP by partners and non-partners.