The　hydrodeoxygenation　(HDO)　of　the　cyclic　five-membered　ester　γ-valerolactone　(GVL)　was　studied　on　a　Ni2P/MCM-41　catalyst.　The　activation　of　the　passivated　catalyst　in　H2　was　followed　by　in　situ　near-edge　X-ray　absorption　spectroscopy　(NEXAFS)　which　indicated　reduction　of　the　catalyst　after　3　h　at　550　°C.　In　situ　infrared　measurements　under　reactive　H2　and　inert　N2　showed　the　presence　of　adsorbed　GVL　and　pentanoic　acid　(PA)　as　the　most　abundant　surface　intermediates.　The　results　supported　a　previous　reaction　network　that　showed　that　ring-opening　of　GVL　to　produce　pentanoic　acid　was　the　rate-determining　step.　This　was　confirmed　by　transient　infrared　measurements　which　showed　that　the　number　of　CH2　groups　in　the　adsorbed　species　increased　under　H2　flow,　consistent　with　PA　formation　but　not　an　alkoxide.　The　results　provide　understanding　of　the　key　steps　in　the　reaction　mechanism.　©　2018　Elsevier　B.V.