Two competing models of uncoupling protein (UCP) transport mechanism agree that fatty acids (FAs) are obligatory for uncoupling, but they disagree about which ion is transported. In Klingenberg's model, UCPs conduct protons. In Garlid's model, UCPs conduct anions, like all members of this gene family. In the latter model, UCP transports the anionic FA head group from one side of the membrane to the other, and the cycle is completed by rapid flip-flop of protonated FAs across the bilayer. The head groups of the FA analogues, long-chain alkylsulphonates, are translocated by UCP, but they cannot induce uncoupling, because these strong acids cannot be protonated for the flip-flop part of the cycle. We have overcome this limitation by ion-pair transport of undecanesulphonate with propranolol, which causes the sulphonate to deliver protons across the membrane as if it were an FA. Full GDP-sensitive uncoupling is seen in the presence of propranolol and undecanesulphonate. This result confirms that the mechanism of UCP uncoupling requires transport of the anionic FA head group by UCP and that the proton transport occurs via the bilayer and not via UCP.