A study on the gas-phase epoxidation of propene with vapor hydrogen peroxide has been carried out. The main purpose was to understand the key factors in the reaction and the relationship between epoxidation of propene and decomposition of hydrogen peroxide, which is the main side reaction. The decomposition was highly influenced by the materials used, being higher in metals than in polytetrafluoroethylene (PTFE) and glass, and it was complete when the epoxidation catalyst, TS-1, was introduced in the system. However, when propene was added, the peroxide was preferentially used for the epoxidation, even with amounts of catalyst as small as 10 mg, reaching productivities of 10.5 kgPO·kgcat–1·h–1 for a gas hourly space velocity (GHSV) of 450¿000 mL·gcat–1·h–1. The hydrogen peroxide was converted completely in all the experiments conducted, with a selectivity to PO of around 40% for all peroxide concentrations. Finally, if concentrations of propene higher than the stoichiometrically required amounts were used, the selectivity to PO increased to almost 90%.