Co/MFI catalysts were prepared by various methods, including wet-ion exchange (WIE), either as such or in combination with impregnation (IMP), solid-state ion exchange (SSI), and sublimation (SUB) of CoCl2 (at 700 degreesC) or CoBr2 (at 600 degreesC) onto H/MFI. The catalysts were tested for the reduction of NOx with CH4 or iso-C4H10 in excess O-2. Below 425 degreesC the SUB catalysts show the highest NOx reduction activity with CH4 or iso-C4H10. Above 425 degreesC, the best performance is given by WIE. Below the temperature of maximum N-2 yield, a mixture of Fe/FER and WIE is superior to either catalyst. Addition of 10% H2O to the feed drastically decreases the N-2 yield in NOx reduction with CH4, but increases the activity with iso-C4H10 under some conditions. Permanent damage of the zeolite lattice as a potential cause for the adverse effect of H2O in the tests with CH4 is eliminated, as the original activity is fully restored after calcination. A 100 h test with a wet iso-C4H10 feed shows excellent stability with a SUB catalyst prepared from CoBr2. Characterization by XRD, H-2-TPR, and FTIR reveals that WIE contains isolated Co2+ and (Co-OH)(+) ions that are only reducible at 700 degreesC. SUB catalysts show additional TPR peaks at low temperature, including a feature at 220-250 degreesC, ascribed to multinuclear Co oxo-ions. The formation of an NOy chemisorption complex is most rapid on these catalysts. No oxidation states between Co-0 and Co2+ are detectable; the one-step reduction of Co2+ to Co-0 clusters could be a cause for the unique propensity of Co/MFI to reduce NOx with CH4. (C) 2001 Elsevier Science B.V. All rights reserved.