This paper aims at studying experimentally speculation as a collusion-breaking device in two different auction mechanisms: the uniform- price sealed-bid auction and the ascending clock auction. Our results suggest that a uniform sealed-bid auction open to speculators should be chosen from a revenue maximization point of view. In this mechanism, compliance agents adopt an aggressive strategy toward speculators. This strat- egy significantly increases the seller’s revenue, compared to the more collusive clock auction. In the latter, on the contrary, bidders accommodate speculators, letting them buy permits in the auction and buying their necessary permits on the secondary market. However, as opening the auction to speculators deteriorates efficiency, the regulator faces a trade-off between these two objectives.