In this note I show that there is a mistake in the proof of uniqueness in Engelbrecht-Wiggans, Milgrom and Weber’s seminal “Competitive Bidding and Proprietary Information” and provide a correct proof.

We study the problem of a regulator who must control the emissions of a given pollutant from a series of industries when the firms' abatement costs are unknown. We develop a mechanism in which the regulator asks firms to report their abatement costs and implements the most stringent emissions...