Erbium films of 250-700 nm thickness were deposited on Mo substrates of different roughness, at different substrate temperatures, for deposition rates of 0.5 nms-1 and 7.65 nms-1 under three different uhv conditions, namely standard, semi-rigorous and rigorous. Microstructures of these films were obtained by SEM, while the grain sizes were measured using an image analyser. The composition of the residual gas atmosphere, before and after evaporation of erbium at different Ts, was investigated and its influence on the structure of films of varying thickness was established. It was found that the grain size increases with substrate temperature and deposition rate and its dependence on substrate temperature is consistent with the activation energy model proposed by Grovenor et al (1984, Acta Metall, 32, 773)1; while at temperatures lower than 675 K, where the number of mobile boundaries, according to Grovenor et al (1984, Acta Metall, 32, 773)1, is limited to regime (b) (Zone T), the deposition rate also has a direct effect on the number of mobile boundaries and increases the effective activation energy directly. Results show that the vacuum condition of the uhv system has a direct effect on the microstructure (growth steps and layering) of erbium films. The poorer the vacuum condition, the more pronounced growth steps and layering will form. Growth steps are apparently associated with larger grains, although it may not be the true sizes of the grains which are measured. Clear Zone II structure is obtained for films of varying thickness deposited under rigorous uhv conditions, using different deposition rates. SEM and nuclear reaction analysis give evidence of diffusion of carbon atoms into the hot film and that this effect may be responsible for the increased prominence of re-nucleation.