The generally accepted three-step mechanism of propene oxidation to acrolein includes abstraction of two hydrogen atoms and addition of one oxygen atom. The first step is a well-known H-abstraction from the methyl group. The sequence of two other steps is unclear. We investigated the reaction in quasi-catalytic mode at 100-200 °C with extraction and analysis of accumulated surface products. The reaction intermediate, surface ether of allyl alcohol, was identified. This strongly proves that O-addition precedes the second H-abstraction. Activation energy and kinetic isotope effect of the quasi-catalytic reaction correlate with appropriate parameters of the conventional catalytic process.