Experiments have been conducted to seek ways to enhance the selectivity of propene oxide in the autoxidation of propene in the gas phase. The experiments were carried out in two systems, one static and the other a continuously stirred tank reactor, in order to obtain a wide range of conditions. The results reported in this paper have been obtained between 505 and 549K and pressures of 0.9 to 55 bar. Monitoring the perturbation in the product distribution shows that the selectivity of propene oxide is increased markedly on decreasing the temperature. Addition of acetaldehyde to propene: oxygen mixtures, by providing an additional source of peroxy radicals, enhances further the yield of epoxide to the extent that the process could be made commercially viable. Addition of formaldehyde was found not to increase the epoxide selectivity, as its oxidation does not provide an additional source of peroxy radicals. A previously published reaction scheme, constructed to model propene oxidation at low pressures (<4 bar) was tested by simulation of the co-oxidation and high pressure experiments reported here.