Market mechanisms aim to deliver environmental services at low cost. However, this objective is undermined by participants whose conservation actions are not marginal to the incentive — or “additional” — as the lowest cost providers of environmental services may not be the highest social value. We investigate this potential market failure in the world’s largest auction mechanism for ecosystem services, the Conserva- tion Reserve Program, with a dataset linking bids in the program’s scoring auction to satellite-derived land use. We use a regression discontinuity design to show that three of four marginal winners of the auction are not additional. Moreover, we find that the heterogeneity in counterfactual land use introduces adverse selection in the market. We then develop and estimate a joint model of multi-dimensional bidding and land use to quantify the implications of this market failure for the performance of environmental procurement mechanisms and competitive offset markets. We design alternative auc- tions with scoring rules that incorporate the expected impact of the auction on bidders’ land use. These auctions increase efficiency by using bids and observed characteristics to select participants based on both costs and expected additionality.