A flow cell was used for the monitoring of the state of copper ions in Cu-ZSM-5 in situ at high temperatures by e.s.r. Reduction of the most reactive square-planar coordinated Cu 2 + ions to Cu + occurs at 500°C in methane flow. The reoxidation of cuprous to cupric ions at 500°C by NO or O 2 is very fast; and, at 500°C, in mixtures of CH 4 with excess oxygen, most of the copper in CuH-ZSM-5 retains the cupric state. The formation of weak adsorption complexes between isolated Cu 2 + cations and CO molecules at 200°C leads to a measurable change in coordination. An activated formation of strong adsorption complexes, with charge transfer, takes place on treatment of the CuH-ZSM-5 by CO at 100-300°C. Subsequent heating in He flow removes all ligands at T > 400°C. At 500°C, in CO flow a slow reduction of Cu 2 + takes place. Cooling of the CuH-ZSM-5 in a mixture [CO + NO] is accompanied by a change in Cu 2 + coordination which is indicative of a mixed adsorption complex.