A flow cell was used for the in situ ESR monitoring of the state and reactivity of chromium ions in Cr-ZSM-5. Calcination of Cr(NO3)(3)/NH4-ZSM-5 in air at 500 degrees C is accompanied by migration of chromium ions inside the zeolitic channels and stabilization of isolated Cr5+ cations near lattice Al3+ ions. Calcination of Cr-ZSM-5 at 750 degrees C leads to a gradual disappearance of the isolated Cr5+ cations and formation of alpha-Cr2O3 microcrystals. All the Cr5+ cations are accessible to gas-phase molecules : O-2 strongly broadens the dipole-dipole signal; H2O sorption increases the local crystal field symmetry; admission of CCl4 results in a small change of the Cr5+ local coordination; strongly stabilized complexes on Cr-ZSM-5 are observed upon sorption of either NO or NO2. The sorption of C3H6 on Cr-ZSM-5 at 20 degrees C is accompanied by a gradual reduction of the Cr5+ sites. At 500 degrees C in [C3H6 + O-2 + He] flow, even at a large excess of oxidant, the reduction of a noticeable part of Cr5+ ions takes place. At 400 degrees C, in the same gas mixtures, a deeper reduction of Cr5+ occurs. Closer to stoichiometric conditions, in a [C3H6 + NO + He] flow with 120% excess of oxidant the Cr5+ in completely reduced at 500 degrees C. The oxidation of propene is accompanied by coke deposition on the surface of the catalyst. The implications of the results are discussed.