Evidence for the lateral diffusion of covalently bound molecules on silica surfaces is reported. A slow growth of pyrene excimer fluorescence was observed from silica samples, sparingly derivatized with (3-(1-pyrenyl)propyl)-dimethylchlorosilane, after exposure of these samples to water. This excimer formation was attributed to the migration of ligands over the surface, despite their continued covalent attachment to the surface. Time evolution of the excimer population was successfully modeled using a simple theory for diffusion-controlled reactions in two dimensions. Diffusion coefficients at room temperature were small (approximate to 10(-20) cm(2)/s), and the encounter distance for excimer formation was reasonable (6 Angstrom), The surface migration appears to proceed through a hydrolyzed intermediate that rebinds to adjacent silanol sites on the silica surface, since the process could be stopped by blocking adjacent sites. The activation energy for migration (16.3 +/- 0.6 kcal/mol) and small preexponential factor are consistent with the barrier to surface diffusion being hydrolysis of the surface siloxane bond.