The mechanism for the formation of monolayers of [Ru(bpy)(2) Qbpy](2+) on platinum microelectrodes has been investigated by monitoring the time evolution of the surface coverage using in-situ voltammetry; bpy is 2,2'-bipyridyl and Qbpy is 2,2':4,4":44"-quarterpyridyl. The monolayers were formed in 2:1 H2O:DMF solution containing between 0.01 and 150 mu M of the complex. The dependence of the surface coverage on the bulk concentration is described by the Frumkin adsorption isotherm in which the free energy of adsorption, Delta G(ads), is -33.8 +/- 1.2 kJ mol(-1) and the interaction parameter is 1.6 indicating destabilizing, surface coverage dependent, lateral interactions. Monolayer formation occurs on the 10-20 h time scale and is controlled by the dynamics of adsorption rather than bulk diffusion. The first-order rate constant for adsorption is 5.1 +/- 0.2 x 10(-6) s(-1). Significantly, the heterogeneous electron-transfer rate constant measured at an overpotential of 50 mV, k50(m) v, is independent of surface coverage but increases from 2.1 +/- 0.3 x 10(5) to 1.4 +/- 0.2 x 10(6) s(-1) if the monolayers are allowed to equilibrate for extended periods. Raman microscopy of modified macroelectrodes indicates that the orientation or binding mode of the adsorbates at equilibrium differs from that found when they first bind to the platinum surface.