Osmium tris-diphenylphenanthroline perchlorate (Os(DPP)(3)) can be spread at the air/water interface where it forms solid monolayer films. Brewster angle microscopy revealed that these films consist of irregular ca. 100 to 1000 mu m diameter 2D aggregates which coalesce upon compression to form continuous films. Crazing incidence X-ray diffraction data showed that the structure of the aggregates is independent of the degree of monolayer compression and features a 2D lattice of hexagonally close-packed Os(DPP)(3) centers with Os-Os distances of 12.57 Angstrom. The latter is 4 to 24% shorter than the Ru-Ru distances found in the 3D monoclinic crystal of an isostructural Ru (DPP)(3). Two dimensional electrochemical measurements carried out with line microelectrodes at the air/water interface were used to study kinetics of the lateral electron transport in these Langmuir monolayers. Electron transport involves electron hopping on the 2D lattice of the osmium sites where the individual electron transfer steps between Os-II(DPP)(3) and Os-III(DPP)(3) take place with the rate constant K-1 = 4.7 x 10(8) s(-1). Percolation theory was used to account for the observed increase of the electron hopping rates during monolayer compression on the water surface resulting in an increase of the extent of connectivity and thus electroactivity of the initially formed 2D Os(DPP)(3) aggregates. Percolation theory also accounts well for the dependence of the electron hopping rates on the composition of fully compressed Os(DPP)(3)/Ru(DPP)(3) monolayers in which the ruthenium species were used to homogeneously dilute the Os(DPP)(3) sites. In contrast, in Os(DPP)(3)/octadecanol monolayers, macroscopic self-segregation of the two components was inferred from a larger positive shift of the apparent percolation threshold in the lateral electron hopping.