The isotope exchange reaction between 13CO and a filament of Ta12C has been studied at filament temperatures between 295 and 2,000°K. The reaction first becomes measurable at temperatures in excess of ca. 1,000°K and is therefore associated with the more strongly bonded of the two states of carbon monoxide on tantalum monocarbide. The efficiency of exchange passes through a maximum of about 2.4 % at a filament temperature of 1,600°K. Of the carbon monoxide desorbed after the filament had adsorbed at room temperature, the lower temperature α peak had not exchanged with the surface, whereas the higher temperature β peak had.