The mechanism for the formation of monolayers of [Os(2,2 ' -bipyridyl)(2) (4,4 ' -trimethylenedipyridine)(2)](2+) [Os(bpy)(2) (p3p)(2)](2+) on gold and platinum microelectrodes has been investigated by monitoring the time evolution of the surface coverage. The monolay ers were formed in 2:1 H2O:dimethylformamide solution containing between 10 and 100 muM of the complex. The voltammetric response of those films corresponding to the Os2+/3+ redox reaction is nearly ideal. Cyclic voltammetry was used to determine the surface coverage vs. time profile following transfer of the modified electrode into aqueous perchlorate electrolyte. Monolayer formation occurs on the tens of minutes time scale and follows a second order rate law for both gold and platinum electrodes. For gold, the rate constant is 4.6 +/- 0.5 x 10(2) M-1 s(-1) for bulk concentrations between 10 and 100 muM. In contrast, more complex behavior is observed on platinum with multilayers being formed where C-B exceeds approximately 20 muM The apparent rate constants for binding to platinum are at least a factor of 2 smaller than for gold. For both electrode materials, desorption follows first order rate kinetics indicating that the adsorbates do not interact strongly with one another. The time constants for desorption are comparable with those found for adsorption. The first order desorption rate constants are 4.1 +/- 0.3 and 1.8 +/- 0.2 X 10(-5) s(-1) modified gold and platinum electrodes, respectively. (C) 2000 The Electrochemical Society. S0013-465 1 (00)08-037-X. All rights reserved.