Interaction of CO with K-FER zeolite was investigated by a combination of variable-temperature IR spectroscopy and computational study. Calculations were performed using omega(CO)/r(CO) correlation method in combination with a periodic density functional theory model. On the basis of agreement between experimental and calculated results, the following carbonyl complexes were identified: (i) mono- and dicarbonyl C-down complexes on single K+ sites characterized by IR absorption bands at 2163 and 2161 cm(-1), respectively; (ii) complexes formed by CO bridging two K+ ions separated by about 7-8 angstrom (dual sites) characterized by a band at 2148 cm(-1); and (iii) isocarbonyl (O-down) complexes characterized by a band at 2116 cm(-1). The bridged carbonyl complexes on dual K+ sites are about 5 kJ/mol more stable than monodentate (monocarbonyl) CO complexes. The C-O stretching frequency of monocarbonyl species in K-FER depends on K+ location in the zeolite, and not on K+ coordination to the framework. A combination of theoretical calculations using a periodic density functional model and experimental results showed formation of two types of monocarbonyls. The most abundant type appears at 2163 cm(-1), and the less abundant one at 2172 cm(-1). These experimentally determined wavenumber values coincide, within +/- 2 cm(-1), with those derived from theoretical calculations.