The reactions of methanethiol (CH3SH) on clean, sulfur-covered, and oxygen-covered Fe(110) have been investigated by X-ray photoelectron spectroscopy, temperature-programmed reaction spectroscopy, and low-energy electron diffraction. On clean Fe(110), the S-H bond in methanethiol breaks below 100 K, affording methylthiolate (CH3Sa) and H-a. Heating to 600 K results in C-S bond cleavage near 290 K, leading to the evolution of methane and dihydrogen, and a p(2 x 2) sulfur overlayer. On the oxygen-covered surface (theta(O) = 0.25 ML), adsorbed methylthiolate and hydroxyl are formed. The amount of irreversible thiol reaction is essentially the same on the O-covered and clean Fe(110) surfaces. The amount of irreversibly bound thiol is reduced by approximately half on the (2 x 2)-S overlayer (Bs = 0.25 ML). The C-S bond in methylthiolate is stabilized more by surface sulfur than by surface oxygen, based on the methane formation temperature shifts of +55 and +20 K, respectively. Oxygen coverages greater than 1 monolayer form an FeO thin film, weakening the interaction of methanethiol with the surface, further reducing the methane yield which now reacts above 400 K.