Inverse photoemission spectroscopy (IPES) was employed to study the density of unoccupied electronic states of fluorinated and non-fluorinated cooper phthalocyanine layers deposited onto hydrogen passivated Si(111) substrates. For the nonfluorinated cooper phthalocyanine (CuPc) the lowest unoccupied molecular orbital (LUMO) is found to shift gradually towards the Fermi level with increasing film thickness. The shift amounts to 400 meV and appears for film thicknesses between one monolayer and 10 nm. This finding complements previous results obtained using ultraviolet photoemission spectroscopy where the highest occupied molecular orbital (HOMO) was found to shift as a function of film thickness. Fluorinated cooper phthalocyanine (F16CUPc) shows the opposite behaviour, that is the distance between LUMO and Fermi level is increasing by 1.2 eV. (C) 2004 Elsevier B.V. All rights reserved.