The oxidation of carbon films on (Sm2O3-gamma)-O-16, exposed to similar to 1 Langmuir/s O-18(2) at 1100 K solely yields (CO)-O-16 while the substrate is quantitatively capturing oxygen. This is evidence that chemisorbed O-18 scrambles its label with the catalyst before oxidizing carbon or diffusing into the bulk oxide. O-diffusion coefficients measured in samaria under present conditions (D-0 greater than or equal to 8 x 10(-9) cm(2) s(-1) above 1000 K) imply that scrambling is complete in a few nanoseconds. We infer that dissociative O-18(2) chemisorption is assisted by the concomitant restructuring of extended surface regions, on which carbon films are oxidized by the statistically preponderant unlabeled oxygen species.