Au/MFI was prepared by sublimation of AuCl3 onto HMFI. The oxidation state of the gold in the as-synthesized sample is Au3+. Upon heating in He or O-2, it transforms into gold metal and electron-deficient gold particles which agglomerate to larger Au particles. Reduction of Au3+ with CO to Au+ is accompanied by formation of an Au+(CO) complex. The carbonyl ligand has a remarkable stabilization effect on Au+ ions in zeolite cages. Strong IR bands show the CO vibration, and also the strong perturbation of the T-O-T vibrations of the zeolite lattice. [Au+(CO)]/MFI has a characteristic XRD pattern and is stable up to 200 degreesC. Au/MFI catalyzes the decomposition of N2O to N-2 even in the presence of 3% O-2. With hydrocarbons or NH3 as the reductant, it has some NOx reduction activity, but these catalysts deactivate at higher temperature even in inert gas atmosphere.