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.