Thin tin-doped indium films were deposited by DC magnetron sputtering at different substrate voltages. The metallic films were annealed within a high-temperature chamber (10(-5) mbar) and studied by in situ grazing-incidence X-ray diffractometry (GIXRD). During the post-deposition annealing, crystalline indium tin oxide (ITO) forms in a diffusion-limited process. A mathematical model was applied to derive the effective diffusion coefficients D-eff from the time dependence of the ITO(222) Xray reflection integral intensity. A strong influence of the bias voltage on D-eff was observed. From the temperature dependence of the diffusion coefficients, the activation energy values for oxygen diffusion into the metallic films were calculated. To understand the differences in D-eff, the microstructure of the as-deposited films was investigated. Information on domain sizes, microstrains and dislocation densities was obtained from X-ray profile analysis using Warren-Averbach and Krivoglaz-Wilkens methods. A distinct change in grain size was found, depending on the negative substrate voltage applied. With respect to the film properties, the contribution of mass transport along grain boundaries, as well as through the lattice and dislocation cores, on the effective diffusion coefficient can be separated. It was shown that D-eff, is mainly influenced by grain boundary diffusion, whereas diffusion within the grains largely determines the activation energy of the process.