Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl2 vapor is deposited. TPR shows peaks for Co2+ ions at 695-705 degrees C and for Co3O4 at 385-390 degrees C, but a peak in the 220-250 degrees C region appears to indicate Co2+ oxo-ions. The catalysts have been tested for the selective reduction of NOx with iso-C4H10 under O-2-rich conditions and in the absence of On, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: ore-ions or Co3O4 clusters first oxidize NO to NO2, which is chemisorbed as NOy (y greater than or equal to 2) and reduced. In this modus operandi catalyst SUB shows the highest N-2 yield similar to 90% near 390 degrees C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co2+ ions and has poor activity below 400 degrees C, excels at T>430 degrees C. This and the observation that, at high temperature, NO is reduced in O-2-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co2+ ions without intermediate formation of NO2. The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO2 formation. (C) 2000 Elsevier Science B.V. All rights reserved.