The ring-opening reactions of propylene oxide (methyloxirane) were studied over an Au/MgO catalyst and Au powder at various temperatures in a pulse microreactor. The supported catalyst was significantly more active, and its activity was dramatically influenced by the method of pretreatment. When it was finished with hydrogen, conversion values were always above 70%, but often it was in the 90%+ range. The major reaction pathways were deoxygenation and isomerization. The comparison in the behavior of Au/MgO and Au powder pointed to the crucial importance of the support-metal interface in oxygen traffic and as a landing place for hydrogen-rich residues being the hydrogen source for water and alcohol formation (hydrogenation).