The formation of electronic states in the fundamental gap at the maleic anhydride/Si(1 0 0)-2 x 1 interface has been studied with photoluminescence (PL) measurements and high resolution electron energy loss spectroscopy (HREELS). We observe that the room temperature adsorption of maleic anhydride on Si(1 0 0)-2 x 1, where the organic molecules are mainly in a di-sigma coordination, results in a gap state density D-it = 1.8(2) x 10(12) eV(-1) cm(-2) which is slightly lower than determined for the clean Si(1 0 0)-2 x 1 surface. After heating maleic anhydride/Si(1 0 0)-2 x 1 to 1115 K, the gap state density has increased to 6.4(6) x 10(12) eV(-1)cm(-2) which we relate to the formation of silicon carbide structures on the silicon surface.