The adsorption/desorption and deposition/dissolution phenomena occurring during the electrochemical transformations of phenazine (Ph) at a gold electrode in aqueous acidic solutions have been studied by cyclic and potential step electrochemical quartz crystal microbalance measurements. Phenazine exhibits two successive one-electron reduction steps in acidic media. In dilute phenazine solutions, the product of the first electron transfer is phenazylium cation radical (PhH2+) which adsorbs in the form of a phenazylium salt (PhH2+ ClO4-, PhH2+ Cl-) at the electrode surface. The perchlorate salt is highly soluble and the electroreduction takes place via a loosely adsorbed state. In chloride containing solutions, multilayer adsorption is observed. The second electron transfer in dilute phenazine solutions results in the formation of 5,10-dihydrophenazine (PhH2) in HClO4, which desorbs from the electrode surface. In HCl solutions, a substantial portion of the fully reduced product, which is a charged dimer, remains on the surface. In more concentrated phenazine solutions in the potential region of the second reduction wave, a deposition process can be observed, which is due to the formation of the quinhydrone-analogue, phenazinehydrine charge-transfer complex. The formation of the charge-transfer complex obeys a second-order kinetics, however, the rate of the film growth is influenced by the simultaneous dissolution process. The increase of the acid concentration enhances the dissolution, and may prevent the film formation. In dilute phenazine solutions, both redox waves are reversible and likewise the adsorption/desorption processes. In concentrated solutions, the reoxidation of the phenazinehydrine film results in a complicated voltammetric response related to a dissolution-redeposition-dissolution sequence. (C) 2004 Elsevier Ltd. All rights reserved.