The　hydrodeoxygenation　(HDO)　of　the　cyclic　five-membered　ester　gamma-valerolactone　(GVL)　was　studied　on　a　Ni2P/MCM-41　catalyst.　The　activation　of　the　passivated　catalyst　in　H-2　was　followed　by　in　situ　near-edge　X-ray　absorption　spectroscopy　(NEXAFS)　which　indicated　reduction　of　the　catalyst　after　3　h　at　550　degrees　C.　In　situ　infrared　measurements　under　reactive　H-2　and　inert　N-2　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　H-2　flow,　consistent　with　PA　formation　but　not　an　alkoxide.　The　results　provide　understanding　of　the　key　steps　in　the　reaction　mechanism.