The　understanding　of　catalytic　mechanisms　is　enhanced　by　the　observation　of　surface　intermediates　at　reaction　conditions　using　spectroscopic　techniques,　but　this　is　insufficient,　as　the　observed　species　may　not　be　involved　in　the　reaction.　This　work　describes　a　general　method　of　analysis　of　hydrogenation　or　oxidation　reactions　which　uses　transient　spectroscopic　data　to　determine　whether　an　adsorbed　species　is　a　reactive　intermediate　or　a　spectator　on　the　surface.　The　assumptions　and　limitations　of　the　method　are　summarized.　Although　the　technique　is　approximate,　it　is　easy　to　implement,　and　provides　order-of-magnitude　estimates　of　the　rate　of　reaction　of　an　intermediate.　The　method　consists　of　measuring　the　change　of　coverage　of　the　species　with　time,　dθ/dt,　during　adsorption　in　inert　gas　or　at　reaction　conditions.　An　example　is　given　with　the　hydrodeoxygenation　of　the　model　compound　γ-valerolactone　(GVL)　using　a　Ni2P/MCM-41　catalyst,　one　of　the　most　effective　catalysts　reported　for　the　transformation.　The　reaction　is　relevant　to　the　upgrading　of　bio-oil　derived　from　pyrolysis　of　lignocellulosic　feedstocks.　The　kinetics　of　the　reaction　and　observation　by　in　situ　infrared　spectroscopy　of　adsorbed　GVL　and　its　transformation　to　pentanoic　acid　are　consistent　with　a　Langmuir-Hinshelwood　mechanism.　Analysis　by　in　situ　transient　X-ray　absorption　fine　structure　shows　that　the　adsorbed　GVL　is　a　true　reaction　intermediate.　©　2020　Elsevier　Inc.