The hydroconversion mechanism of gamma-valerolactone (GVL) was studied over a Co/SiO2 and a Pt/aluminosilicate catalyst. The reaction was carried out at 250 degrees C, 30 bar, and WHSV = 1 g(GVL).g(cat).(-1) h(-1). The Co/SiO2 catalyst had moderate hydrogenation activity and Lewis acidity, whereas the Pt/aluminosilicate catalyst had high hydrogenation activity and Bronsted acidity. Diffuse Reflectance Fourier Transform Spectroscopic (DRIFTS) results suggested that the GVL ring was bounded more strongly to the stronger acid Pt/aluminosilicate than to the weaker acid Co/silica catalyst. The Pt/aluminosilicate catalyst was substantiated to open the GVL ring in a protonation/deprotonation process giving pentenoic acid (PE) intermediate and pentanoic acid (PA) as main final product. Over Co/SiO2 catalyst 2-methyl-tetrahydrofuran (2-MTHF) and pentanol were the major products of GVL conversion. It was substantiated that latter transformation proceeded in consecutive hydrogenation and dehydration steps via 2-hydroxy-5-methyl-tetrahydrofuran and 1,4-pentanediol (1,4-PD) intermediates. The oxygen atoms of GVL were shown to establish H-bonds with the silanol groups of the Co/SiO2 catalyst. The nu(CO) frequency of the adsorbed GVL depends on the adsorption interaction of the GVL and the silica surface. Three distinct nu(CO) bands were distinguished by DRIFTS. Quantum chemical calculations gave the structures of the three adsorbed GVL species. Operando DRIFTS examination of the catalytic reaction suggested that in the structure that was activated for hydrogenation/hydrogenolysis both the ring and the carbonyl oxygen were bound to silanol groups.