In this paper we present photoemission studies of the influence of 12-hour exposure to the ambient air on the chemical and electronic properties of thin 16-nm copper phthalocyanine (CuPc) sensing layers deposited on nand p-type silicon Si(111) substrates covered with the native oxide. The surface chemistry and electronic parameters of organic thin film including surface band bending, work function, electron affinity and their variations upon the exposure have been monitored with X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy techniques. We found that after the exposure, the surface chemistry of CuPc remained unaffected, however the work function and surface band bending increased by 0.55 eV and 0.45 eV for the layers on n-Si and by 025 eV and 0.30 eV for those on p-Si. Additionally, we detected a slight surface dipole at CuPc on n-Si manifested by a small shift in electron affinity of 0.10 eV. In order to explain these changes we developed a model basing on the interaction of ionic species with the phthalocyanine surface. (C) 2010 Elsevier B.V. All rights reserved.