Nucleation kinetics of UHV deposited erbium on amorphous carbon at room temperature and for atom arrival rates of 1.5 × 1013-1.8 × 1014 atoms/cm2 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 × 1014 atom/cm2 s), the saturation density is constant and independent of R but at higher rates it increases with R according to the mathematical function ns = a + bexp(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 xs = (1.53±0.01) nm, and the difference between adsorption and surface diffusion energies, Ea - Esd = (0.12±0.01) eV are estimated.