The adsorption, desorption, and thermal decomposition of acetylene and ethylene on the Si(100)-(2X1) surface have been investigated with emphasis on the modifications induced by coadsorbed hydrogen. Based on high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption spectroscopy (TPD), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES), we show that the adsorption of acetylene and ethylene is blocked by preadsorbed hydrogen leading to a hydrocarbon saturation coverage which decreases linearly with hydrogen precoverage. At low temperatures preadsorbed hydrogen has no influence on the surface chemistry of acetylene or ethylene. At approximately 550 K, coadsorbed hydrogen induces decomposition of ethylene which is not observed in the absence of hydrogen. After postexposures of an ethylene-saturated Si(100)-(2X1) surface to gas-phase atomic hydrogen with fluences below 5x10(14) cm(-2), the ethylene is essentially unperturbed at low surface temperatures with partial decomposition upon heating as for preadsorbed hydrogen. Higher postexposures of atomic hydrogen lead to Si-C bond cleavage and the formation of ethyl. The desorption of molecular ethylene is then up shifted by approximately 100 K. The experimental results and observed reaction intermediate are explained by an elemental adsorption and reaction model.