Pressure-area diagrams and Brewster angle microscopy of osmium(II)-tris-4,7-diphenylphenanthroline perchlorate (Os(DPP)3) monolayers at the air/water interface (0.05 M HClO4) revealed the existence of 2-D solid aggregates. Their compression produces structurally homogenous films of densely packed Os(DPP)3 centers. Electrochemical experiments carried out directly at the air/water interface using "line" electrodes generated diffusion controlled current due to the lateral electron hopping on the 2-D network of the Os(DPP)3 centers. Voltammetric current was interpreted in terms of the unimolecular rate constant of electron transfer (k1 almost-equal-to 5 x 10(8) s-1) involving neighboring donor acceptor sites. In subsequent experiments, addition of small quantities of a polar solvent (acetonitrile, benzonitrile, nitrobenzene, dimethylformamide or N-methylformamide) to the subphase led to a complete resolvation of the Os(DPP)3 monolayer and resulted in changes in the electron transfer rate constant. Analysis of these data in view of the Marcus theory showed a good correlation between the observed nuclear frequency factor and the inverse longitudinal relaxation time of the solvents used in the solvent exchange experiments.