The adsorption and thermal decomposition of alkanethiols (R-SH, where R = CH3, C2H5, and C4H9) on Pt(111) were studied with temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. Dissociation of sulfhydryl hydrogen (RS-H) of alkanethiol results in the formation of alkanethiolate; the extent of dissociation at an adsorption temperature of 110 K depends on the length of the alkyl chain. At small exposure, all chemisorbed CH3SH, C2H5SH, and C4H9SH decompose to desorb hydrogen below 370 K and yield carbon and sulfur on the surface. Desorption of products containing carbon is observed only at large exposure. In thermal decomposition, alkanethiolate is proposed to undergo a stepwise dehydrogenation: R'-CH2S -> R'-CHS -> R'-CS, R' = H, CH3, and C3H7. Further decomposition of the R'-CS intermediate results in desorption of H-2 at 400-500 K and leaves carbon and sulfur on the surface. On the basis of TPD and XPS data, we conclude that the density of adsorption of alkanethiol decreases with increasing length of the alkyl chain. C4H9SH is proposed to adsorb mainly with a configuration in which its alkyl group interacts with the surface; this interaction diminishes the density of adsorption of alkanethiols but facilitates dehydrogenation of the alkyl group.