The adsorption of carbon monoxide on a platinum-rhodium alloy (111) single crystal surface was studied with a combination of reflection absorption infrared spectroscopy, thermal desorption spectroscopy, and work function change measurements. It was found that CO shows a pronounced preference for adsorption on Rh sites both at 100 K and room temperature, up to a coverage at which available next-nearest neighbor Rh sites were filled. Adsorption was found to be predominantly in atop sites and only weak bridge bands were detected at 100 and 300 K.