Introduction Considerable efforts have been made during the last two decades in order to understand the electronic properties of phthalocyanine (Pc) compounds, well-known organic materials with interesting semiconducting [ 1 to 31 and catalytic [4] properties. Because of both high temperature and high chemical stability [5], they became widely used in thin film technology for specific optoelectronic devices [6], solar cells [7], and gas sensors [8].As for the most popular copper phthalocyanine (CuPc) thin films, up to now only UPS [9 to 131, SPS [14, 151, and EELS [16] methods have been used to study their electronic properties. In the earliest UPS studies of Vilesov et al. [9], reexamined by Schechtman and Spicer [lo], only the optical density of states in the upper part of the valence band up to -7 eV has been determined. It was characteristic of the conjugated inner ring of the main molecule and found to be in rather weak agreement with the free-electron molecular orbital calculations [ll]. Thereafter, Pong and Smith [12], and Hoechst et al. [13] have studied the copper phthalocyanine (CuPc) photoemission spectra for the photon energy range of 7 to 23 eV, but only for the higher photon energies they observed the structure characteristic of the optical density of states. In addition, the electron mean free path for energies larger than 1.5 eV above the valence band was estimated to be about 1 nm [12]. The electronic states near the top of the valence band have also been observed in the surface photovoltage spectroscopy (SPS) experiments of Dahlberg and Musser [14, 151 as well as in electron loss spectroscopy (EELS) studies of Tada et al. [16].Because of the limited energy range and sensitivities of the above-mentioned conventional methods, many questions concerning the electronic properties of the copper phthalocyanine (CuPc) thin films still remain unanswered. Investigations using other techniques are expected to give new and complementary information.One of the best and suitable methods for this purpose seems to be photoemission yield spectroscopy (PYS), developed by the group of Sebenne in Paris [17]. It consists in measuring the total number of photoemitted electrons per incident photon at a given light energy E (usually up to 6.2 eV). Because of the large dynamic range (up to eight orders of magnitude) and high energy resolution (up to 0.02 eV), PYS allows the determination, among others, of the work function cp and the ionization energy @. Moreover, with the debatable, but