Nucleation kinetics of UHV deposited erbium on amorphous carbon at room temperature and for atom arrival rates of 1.5 × 10 13 -1.8 × 10 14 atoms/cm 2 s is studied. For all deposition rates, the density of nuclei increased with time to a saturation value and remained stationary. It was found that the nucleation rate is linearly dependent on the deposition rate, indicating that a single-adatom constitutes a stable nucleus. At lower deposition rates (up to R = 1 × 10 14 atom/cm 2 s), the saturation density is constant and independent of R but at higher rates it increases with R according to the mathematical function n s = a + b exp(R/c). It is shown that the nucleation theory usually used to explain the experimental work needs some revision. For measured condensation coefficients, the diffusion length of adatoms x s = (1.53 ± 0.01) nm, and the difference between adsorption and surface diffusion energies, E a − E sd = (0.12 ± 0.01) eV are estimated.