The influence of the preparation temperature on the surface acoustic wave (SAW) propagation velocity of iron thin films deposited by DC magnetron sputtering has been studied by High Resolution Brillouin Spectroscopy (HRBS). A clear different elastic behaviour has been found for sample grown at 200 K substrate temperature. Numerical simulation has been used to reproduce experimental elastic data and it is possible to relate the difference in elastic response in different samples to the existence of a FeO layer among other oxides. This effect is discussed in relation with the iron oxidation type that also affects the magnetic properties of iron thin films on silicon substrates.