Adsorption of CO onto dehydrated cobalt-exchanged ZSM-5 zeolite was studied by CW-EPR techniques. It is shown that the reversible formation of a low spin carbonyl η1{Co(CO) n }7 adduct upon addition of carbon monoxide at p CO>50–60 Torr involves significant change in the spin state of Co2+ from S=3/2 to 1/2. The spin Hamiltonian parameters of the adduct g x =2.222, g y =2.184, g z =2.011, |A x |=3.8 mT, |A y |=3.2 mT, |A z |=7.9 mT were determined by a computer simulation and further discussed in terms of the possible ground state and molecular structure. It is shown that the η1{Co(CO) n }7 cage complex exhibits a C2v symmetry with |z 2,2A1〉 ground state.