The electronic properties of the space charge layer of copper phthalocyanine (CuPc) prepared under ultrahigh vacuum (UHV) conditions were in situ studied by photoemission yield spectroscopy (PYS). Depending on the oxygen exposure, as well as subsequent UHV annealing the variation of the work function phi, ionization energy Phi and the surface Fermi level position in the band gap EF-Ev were analyzed, together with the effective density of filled electronic states localized in the band gap below the Fermi level E-F. Moreover, the obtained results were compared to the recently determined ex situ studied CuPc thin films.