Thioalkanoic acids form highly ordered monolayer assemblies upon adsorption on surfaces of evaporated silver and gold. The thiocarboxyl group coordinates through a similar polar covalent bond to the surface as in the case of an alkanethiolate monolayer but exhibits short term stability. Hydrolysis of the thiocarboxyl head group proceeds at the interface to give the corresponding oxy acid and metal sulfide. On a silver surface, the resulting carboxylic acids react with surface oxide and transform into a closely packed alkanoate monolayer, while on a gold surface, the acid molecules rearrange to form discrete crystallites of H-bonded dimer, together with some anhydride formation. The alkyl chains of the crystallites aligned nearly parallel to the surface. Atomic force microscopy shows that the clusters are of nanometer scale dimension.