The adsorption behaviour of Si(111)2 x 1 and Si(111)7 x 7 surfaces towards Fe(CO)5 has been investigated in the temperature range 80-300 K and in the exposure range 0.1-1000 L. For both surface reconstructions, the adsorption state is found to be molecular at 80 K, from a comparison with the already reported spectrum for pure Fe(CO)5, in the gas phase, while there is partial decomposition at room temperature. The molecular adsorption state rapidly evolves into a partially decomposed one, corresponding to the adsorption spectrum at room temperature. The decomposition is demonstrated to be photoinduced.