We present a resonant inelastic x-ray scattering ~RIXS! study of a strongly bound adsorbate on a semiconductor surface, C2H4 /Si(001). The valence electronic structure as well as the photon energy dependence in RIXS can be studied without the dominating effect of dynamic metallic screening. We demonstrate that for this strongly coupled system the RIXS spectrum resulting from a selective excitation into the unoccupied s CSi * resonance can be interpreted with the help of density-functional calculations. In addition, we show how excitation into different resonances leads to a significant photon energy dependence of the RIXS spectral features, not seen in strongly coupled adsorbate systems on metals.