Copper exchanged zeolites are proving to be very important in decomposing NO into its elements. This work compares the activities of under and over exchanged copper ZSM-5 catalysts and their properties when treated in reducing atmospheres. The copper in the under exchanged samples was shown to be relatively stable to reduction in reducing atmospheres. However, the copper in over exchanged Cu-ZSM-5 was easily reduced and seemed to migrate forming aggregates on the surface detectable by XPS. This aggregate formation was shown to be reversible. A proposal on the state of copper within the zeolite is presented.