We investigated Cs on copper phthalocyanine using photoemission spectroscopy. We observed valence and core level spectra change at different Cs to CuPc ratios, and found that the doping induces energy level shift that can be seen as in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from Cs to CuPc, and the second stage is characterized by the significant modification of CuPc energy levels. The energy level shift in the first stage depends in a semi-logarithmic fashion on the doping concentration. (C) 2003 Published by Elsevier B.V.