The redox chemistry of Co ions in cavities of zeolite MFI has been studied in materials prepared by solid-state ion exchange and displaying Co/Al ratios varying from 0.4 to 1.0. Fourier Transform Infrared (FTIR) spectroscopy, electron spin resonance (ESR), and diffuse reflectance UV-visible spectroscopy were used to identify the oxidation state and the coordination of the Co ions. In the dehydrated blue material, Co2+ ions in cationic exchange sites strongly interact with the zeolite framework; their coordination symmetry is tetrahedral. ESR reveals that the ions are in their high-spin state, detectable only below 60 K; no ESR signal was detected at 77 K. However, adsorption of ammonia ligating to the Co2+ cations gives rise to an ESR signal at 77 K, indicating a strong reconfiguration of the electronic states. after coadsorption of oxygen and ammonia, ESR reveals the presence of two kinds of cobalt: Co2+ in a high-spin state at g = 5.1, detected at 77 K, and low-spin (Co3+LxO2-)(2+) adducts inside the zeolite cavities at g = 2.0, where L = NH3 and probably x less than or equal to 4. The (Co3+LxO2-)(2+) complexes are thought to be located inside the main channel intersections where a large number of ammonia ligands can be attached to a Co2+ ion. The high-spin Co2+ ions at g = 5.1 are assumed to be located in small cavities with five- and six-membered rings, and carry a smaller number of ammonia ligands.