We found that dibenzyl viologen (dBV) on an HOPG electrode undergoes a two-step first order faradaic phase transition at high concentrations of bromide ion (Br-). Results of voltammetric and electroreflectance measurements were used to describe the mechanism of the two-step transition processes. When [Br-] > 180 mM, the transition step at less negative potential was ascribed to a phase transition between a gas-like adsorption layer of dBV dication (dBV(2+)) and a mesophase of dBV radical cation (dBV(center dot+)). Most likely, the mesophase is a two-dimensional (2D) ordered phase composed of co-adsorbed dBV(center dot+) and Br- where both are in direct contact with the HOPG surface. The transition step at more negative potential was ascribed to a phase transition between the dBV Br- mesophase and a 20 condensed phase of dBV(center dot+). In the condensed phase being denser than the mesophase, dBV(center dot+) molecules are pi-stacked due to face-to-face interaction between bipyridinium radical cations. This transition step involves also a reduction process of dBV(2+) to dBV(center dot+) followed by its incorporation into the condensed phase. The two-step transition was not observed in KCl solution of any concentration, either in KBr solution of [Br-] < 75 mM. Other viologens examined, including benzyl-heptyl viologen, did not exhibit such a two-step transition but single-step one. The nature of the transition, especially in the [Br-] range from 75 to 180 mM, was closely analyzed. (C) 2013 Elsevier Ltd. All rights reserved.