We grow homoepitaxial stoichiometric TiN(001) layers by ultra-high vacuum reactive magnetron sputtering in Ar/N 2 mixtures and use scanning tunneling microscopy to study nucleation as a function of the N 2 gas fraction fN2 and growth temperature T s . The characteristic island size R c necessary to nucleate a new layer decreases continuously with fN2, varying from 18.0 nm at T s =740 °C with fN 2 =0.10 to 11.2 nm with fN2=1.00. Over the temperature range 600⩽T s ⩽860 °C, nucleation is diffusion limited with an activation energy E s of 1.1±0.1 eV for TiN(001) growth with fN2=0.10 and 1.4±0.1 eV in pure N 2 . We attribute the increase in E s to a higher steady-state N coverage resulting in an increase in the average x-value of the primary surface-diffusing species, TiN x admolecules.