The interface formation between the molecular beam deposited molecular semiconductor, 3,3,9,10-perylenetetracarboxylic dianhydride (PTCDA), and Se-treated GaAs(100)-(2 x 1) surface was investigated by soft X-ray photoemission spectroscopy (SXPS) using synchrotron radiation. For room temperature growth, no chemical bond is observed. At elevated sample temperature up to 350 degrees C after the growth, all PTCDA except one monolayer desorbs from the surface. A loss of Se dimer atoms on top of the surface, which is in accordance with the number of anhydride oxygen atoms of PTCDA in a unit cell, indicates the reaction with PTCDA molecules upon annealing. The growth mode and the band banding upon PTCDA deposition on Se-treated GaAs(100) are also discussed.