Coadsorption of ethanethiol (C2H5SH) with sulfur, oxygen, and water on the Fe(100) surface under ultrahigh vacuum has been investigated using temperature-programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). Precovered sulfur shows a site-blocking effect for ethanethiol adsorption on the Fe(100) surface. Oxygen also shows a similar behavior but is less effective than sulfur. The reactivity of ethanethiol is reduced by the presence of preadsorbed sulfur or oxygen as indicated by the increasing decomposition temperature with increasing sulfur or oxygen coverage. Ethanethiolate is formed on adsorption, and its decomposition is limited by C-S bond cleavage followed by ethylene and ethane formation. Water physically adsorbs on the ethanethiolate overlayer and does not affect the adsorption and decomposition of the thiolate. However, strong interaction between iron and ethanethiolate results in a displacement of chemisorbed water from a water precovered surface into a physisorbed state at 100 K.