This analysis of the Mexican Treasury securities primary auctions suggests that the uniform format yields higher revenues than the discriminatory format. It applies the structural econometric model proposed by Février, Préget, and Visser (2004). This model's main advantage is that it allows us to (i) estimate the parameters that characterize the distribution function of the securities'marginal value and the conditional distribution of the signals given the securities'value; (ii) derive optimal bids and equilibrium prices of alternative auction mechanisms; and (Hi) compare revenues. The uniform format's revenue superiority seems to be due to market uncertainty, defined as an environment with noisier value signals.