Even when efficiency gains can be expected from contracting out municipal services, it surprisingly rarely occurs. Political pressure by interest groups, especially unions, is usually assumed responsible. We argue that the effectiveness of such pressure is actually a function of a service’s “transparency,” which determines voter’s knowledge of the quality and cost of the service as well as the tax burden. A model is presented of the interplay between union opposition to contracting and service transparency. Policies are offered to increase transparency and make contracting optimal for governments.