The gas-phase epoxidation of propylene using nitrous oxide as an oxidant over bulk and supported iron-containing catalysts was studied. The presence of silica and an alkaline metal in both types of catalysts was found to be indispensable. Bulk catalysts formed in potassium-iron mixed silicates were studied, with respect to their crystalline structure, oxidation state and performance in propylene epoxidation. Over these catalysts a 69% selectivity to propylene oxide at 1.6% propylene conversion was reached. The presence of separated iron oxide phase was shown to promote the undesired allylic oxidation of propylene. Supported, halide-modified Fe-containing catalysts exhibit a superposition of two epoxidation routes. One, independent of air-pretreatment, shows similarity with that observed at bulky potassium-iron silicates; and the second, more selective, induced by air-pretreatment. The second mechanism, dominant in the first 30 min on stream, results in a maximum of 45.7% propylene conversion at 70% selectivity to propylene oxide. An improvement of the epoxidation activity was obtained, comprising reaction-reactivation switching steps. A reaction mechanism involving KCl as an oxygen transmitter and a metastable form of iron moieties is suggested. (c) 2008 Elsevier B.V. All rights reserved.