Rhodium(III) ions have been incorporated in ionic domains of sulfonated linear polystyrene (PSSA) to form PSSA-RhIIIfilms. The Rh-containing domains can act as isolated chemical reactors, and their chemical interactions have been explored in this work by controlling the exposure of the rhodium sites to CO, H2, and H20 under mild conditions. Reaction with CO at low temperature, without either prior reduction by H2 or complete dehydration of the PSSA-RhIIIfilms, results in production of a Rh1(CO)2species. Partial or complete reduction of the films with H2preceded or followed by exposure to CO results in the formation of Rh1(CO)2and, under various conditions, linear and bridging PSSA-Rh0carbonyls and CO bound to small Rh(0) particles. The implications of the results for catalysis are discussed.