The interaction of HCN (DCN) with Si(111)-7X7 was studied with high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). HCN (DCN) formed dimers and/or polymers on the surface at 100 K with higher dosages (D > 4 langmuirs). Above 200 K, CN radicals in an end-on adsorption geometry were identified as the major species which survived up to 600 K on the surface. This observation was different from that with HCN (DCN) on Si(100)-2X1, where the relative concentration of the HCNH species was higher. The observed difference in the behavior of HCN (DCN) on Si(111)-7X7 and Si(100)-2X1 could be ascribed to the different topologies of the two surfaces.