The surface sensitivity of soft x-ray photoelectron spectroscopy is exploited to probe the structural, electronic and chemical properties of an organic/inorganic semiconductor interface. Thin films of tin-phthalocyanine (SnPc) are deposited on a GaAs(001)-1 × 6 surface, prepared by argon ion bombardment. Core level photoemission spectra are analysed to follow the evolution of the interface with SnPc coverage. The results indicate that the interface is largely chemically inert, while the overlayer growth mode is closer to Stranski-Krastanov. Valence spectra show the same molecular features throughout the coverage range. The valence band offset and the interface dipole of this heterojunction are 0.45 and -0.37 eV respectively. The interface dipole may have its origin in the difference in electron affinity of the organic and inorganic semiconductors.