Electron transport properties reflect both the size-dependent electronic structure of metal ultrathin films and their morphology. We investigate these transport properties by means of IR-transmittance spectroscopy performed during growth of iron on MgO(001) at room temperature. Careful analysis of IR-spectra allows us to detect the onset of metallic conductivity due to percolation of island-like metal films. Based on a model for the dielectric properties of metallic thin films, the IR-optical data yield a measure for the thin-film dynamic conductivity. We show that its extrapolation to the static limit provides reasonable values for the d.c.-conductivity of the growing films.