Subject of the present work is the diffusion of adsorbed particles on single crystal planes, which is investigated by scanning tunneling microscopy (STM) and analyzed quantitatively. By evaluation of the atomic motion of oxygen atoms on Ru(0001) it is demonstated, how the transition from the microscopic to the macroscopic description of diffusion can be realized experimentally. Computer-aided image processing permits the statistical evaluation of long sequences of several thousands of atomic configurations, which were recorded with a fast STM system that allows frame rates of up to 20 frames per second. On the one hand, this enables the measurement of the jump rate of isolated oxygen atoms at room temperature as well as the influence of the mutual O-O interaction on that rate over distances of up to 3 lattice constants; from these data the pair potential can be derived. On the other hand the chemical diffusion coefficients were directly determined at various coverages by analysis of the particle number fluctuations. These are found to be in agreement with the microscopic jump rates under consideration of the mutual O-O interaction. Additionally, variable-temperature measurements on the diffusion of sulfur on Pt(111) with coadsorption of CO are reported. For the diffusion of sulfur an activation energy of 550 meV is found. Coadsorption of CO leaves this activation energy unchanged, however, the prefactor is increased by two orders of magnitude by coadsorption of half a monolayer of CO. Various models for this behavior are discussed
