The phase transition of an adsorption layer of heptyl viologen (HV) at a basal plane HOPG electrode was investigated using double potential step chrono-reflectometry in the visible wavelength region. For the first order faradaic phase transition between an expanded adsorption layer of oxidized form (HV2+) and a two-dimensional (2D) condensed monolayer of reduced form (HV.+), the nature of the bi-stable potential region was targeted to be explored. The merit of chrono-reflectometry capable of directly tracking the change of 2D superficial fractions of the two phases on the electrode surface was demonstrated experimentally. The double potential step measurements made it possible to gain access to the steady-state coexistence of the two phases in the bi-stable potential region. After the secondary reverse potential step to the positive direction into the bi-stable potential region was given from a transient nucleation state at the first step potential, the superficial fraction of the condensed phase monitored by reflectance was saturated. The plot of this saturated value (phi(dbl)) as a function of potential (E) in the bi-stable potential region was found to be dependent on the time period spent at the transient nucleation potential. Therefore, the phi(dbl)-E curve thus obtained does not necessarily represent a unique, true equilibrium relationship. (C) 2004 Elsevier B.V. All rights reserved.