There has been considerable recent interest in the valuation of ecosystem services. We focus here on the value of such services in the production of market goods. Although the conceptual basis for conducting such exercises is straightforward, the data with which to implement them empirically is generally not available. An upper bound on the value of ecosystem services arises when the production technology exhibits constant returns to scale in ecosystem services and market inputs jointly. There are compelling reasons to suppose that the existence of fixed factors of production would imply that production technologies exhibit decreasing return to scale. Under these circumstances, no general conclusions can be drawn. We show in an illustrative example that a range of outcomes is possible, depending on the substitutability between ecosystem services and other inputs and the scarcity of ecosystem services relative to other factors of production.