The homogeneous gas-phase reaction of propane and propene with molecular oxygen was investigated at 360-520 degrees C under atmospheric pressure using a quartz reactor in a continuous-flow system. At a high concentration (65 vol %) and low conversions of hydrocarbons, the overall oxidation rate and the selectivity to propylene oxide were markedly enhanced by mixing propane and propene. At a specific propane/propene ratio, for example 10:3, the net conversion of propene was found to become zero at around 450 degrees C, indicating that the consumption of propene was compensated by the formation from propane and only propane substantially reacted to form propylene oxide and the others. The major product was propylene oxide, and its selectivity exceeded 30%. Other products were ethylene, CO, acetaldehyde, methane, CO2, acrolein, propanal, methanol, and so forth.