The thermal decompositon of s-triazine (ST) on Si(100)-2×1 was studied with HREELS, UPS, XPS, and TDS. Two desorption peaks at 180 and 235 K were noted in the TD spectra due to the overlayer and the mono- or submonolayer adsorbates, respectively. Above 200 K, the ST is likely to lie flat on the surface as indicated by the virtual absence of the asymmetric ring breathing mode at 145 meV in HREELS and by the relatively lower ST coverage (<0.3 L) at Ts > 200 K. Annealing a 1.2-LST-dosed sample at 550 K caused the dissociation of ST into HC=N, accompanied by the formation of NH and SiH bonds. Further annealing the surface at 780 K dissociated the CN-containing species and the NH bond. Above 800 K, a mixture of silicon nitride and silicon carbide was formed on the surface following the cracking of the CH bond and the desorption of the H species. The photodissociation of ST on Si(100)-2×1 was also examined; CN radicals and the HCN species could be identified on the surface after it was exposed to 308-nm excimer laser radiaton at 100 K. The appearance of CN radicals reflects the difference between the thermal decomposition reaction and the photodissociation process.