One advantage of replacing traditional-style monopoly regulation by franchise bidding are its low information requirements. Without precise information on demand and cost conditions, it is, however, im- possible for an auctioneer to determine the welfare-maximizing fran- chise duration. In order to solve this problem, we develop a dynamic revelation mechanism which aims to …nd the optimal contract duration in repeated auctions by using potential bidders’knowledge. We devise a mechanism which requires interested …rms to reveal their estimates of the welfare-optimal contract duration. Announcing and establish- ing 10 rules leads to truth revelation when it is common knowledge that …rms are identical. In all other cases, the mechanism restricts the deviations from the optimum. We show that truth revelation is costly unless the prior beliefs of the regulator are accurate.