The adsorption and desorption of n-alkanethiol monolayers on Au(111) have been studied under ultrahigh-vacuum condition by the use of scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and Auger electron spectroscopy (AES). Molecularly resolved STM observations for the alkanethiol monolayers have revealed that at least four different phases evolve during growth, which results in a multistep growth of the monolayer. The desorption species drastically changes at a critical coverage, which is accompanied by a structure change from a low-density flat-lying phase to a denser standing-up phase: While the latter phase bimolecularly desorbs as disulfides, the former phase unimolecularly desorbs as thiolate radicals. The coverage-dependent change of the desorption mode is explained in terms of the difference in the molecule-substrate bonding.