In situ infrared spectroscopy is used to gain a deep understanding of the surface reactions during the hydrogenolysis of tetrahydrofurfuryl alcohol vapor on Ru/SiO2 and SiO2 and to identify intermediates that lead to catalyst deactivation. Time‐resolved in situ infrared spectroscopy experiments elucidate the formation and consumption of different surface species. Hydroxy valeraldehyde is the key intermediate and can undergo hydrogenation or the Tishchenko reaction. Both reactions yield 1,5‐pentanediol, but the latter also produces hydroxy valeric acid, which can polymerize on the catalyst surface. Finally, hydroxy‐valeraldehyde can also participate in fouling by means of aldol condensation. The same reaction intermediates are found both on Ru/SiO2 and SiO2 suggesting that the support plays a role in retaining the molecules on the surface and favoring the multi‐step reaction mechanism on the surface rather than the direct ring opening mechanism.

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