A study of the formation of an interface between the archetype organic semiconductor, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), and GaAs(100) is presented. We examine the growth of thin (few monolayers) and thick (up to 3000 angstrom) organic layers are examined at room and low temperature, and conclude that bonding across the PTCDA/GaAs interface is stronger than the PTCDA van der Waals intermolecular bond. Although the molecular stacking distance of 3.21 angstrom is clearly seen in x-ray diffraction, the films are polycrystalline, and no interface molecular ordering is detected. The interface electronic structure and the energy band lineup are studied with photoelectron spectroscopy. The highest occupied molecular orbital in PTCDA is found at 0.7 eV below the top of the GaAs valence band, in agreement with a vacuum level alignment a this interface.