The characteristic CO vibrational frequency of supported monocarbonyl complexes (RhCO)-C-1, at 2014 and 1984 cm(-1) on dealuminated Y zeolite and alumina, respectively, is lower than the frequencies of both the symmetric and the antisymmetric CO normal modes of the corresponding stable supported Rh(I) dicarbonyls. The CO mode with a measured frequency between those of the symmetric and antisymmetric CO frequencies of the dicarbonyls, previously assigned to rhodium monocarbonyl, is reassigned to mixed carbonyl dihydrogen complexes Rh(H-2)(CO) or Rh(H)(2)(CO). This reassignment is based on a critical analysis of reported experimental data, supplemented by quantum chemical calculations.