The mechanism of transport of sulfuric acid through an anion exchange membrane is studied on the basis of the proton transport number, theta(H+) called proton leakage. The comparison of theta(H+) values between HCl and H2SO4 solutions shows that this proton leakage is always greater for H2SO4 than for HCl. Taking into account relatively close values of water content for the membrane in contact with these two acids, the difference of theta(H+) is explained by different mechanisms of transport for Cl- ions and sulfate species. A specific mechanism of proton transport is presented, which takes into account the nature of the sulfate ion which behaves as a proton acceptor mediating the proton leakage.