The thermal and photochemistry of ethyl iodide, C2H5I, on Rh(111) surface has been investigated by temperature-programmed desorption, high-resolution electron energy loss, X-ray photoelectron, and Auger electron spectroscopies. C2H5I adsorbs dissociatively at submonolayer coverage at 90 K and molecularly at higher coverages. The dissociation of a monolayer starts above 170 K and completes below 250 K. Illumination of adsorbed C2H5I enhanced the extent of the dissociation which is ascribed to the generation of photoelectrons. The primary products of thermal dissociation are adsorbed C2H5 and I. The species C2H5 is stable on the Rh(111) surface up to 200 K : a fraction of C2H5 undergoes dehydrogenation to C2H4 at 200-300 K, and another fraction hydrogenates into C2H6. Dimerization of ethyl species produced by thermal dissociation was not observed. It occurred, however, following the photoinduced dissociation of ethyl iodide at 90 K. Coadsorbed CO induced the desorption of more strongly adsorbed C2H5I, inhibited the C-I bond breaking, and enhanced the stability of the C2H5 species on Rh(111).