Silicon surfaces with different reconstruction, Si(111)-7 7 and Si(001)-2 1, are exposed to a molecular beam of copper phthalocyanine. On Si(111) the adsorbate-substrate interaction is strong, as suggested by a partial reduction of copper shown in the XPS spectra and by the evidence provided by STM of a chemical reaction involving some of the Si adatoms. As a consequence, the molecules are not imaged at atomic scale by STM. At variance with this behaviour, on the single-domain Si(001)-2 1 face, the adsorbed molecules are clearly imaged by STM, suggesting an adsorption state more weakly bonded, This is the first observation of Cu phthalocyanine molecules imaged on a silicon surface, and it further illustrates the potential of STM for surface chemistry.