This work is a contribution to the understanding of the effect of spillover hydrogen in hydrotreating catalysts. Mixtures of sulfided CoMo/SiO2 and an amorphous silica-alumina were studied. The hydrocracking of diphenylmethane was used as a test reaction. The addition of CoMo/SiO2 to silica-alumina strongly increases the hydrocracking rate (by a factor between 4.5 and 8) and the OH-OD exchange (by a factor between 2.5 and 10 after a 20 h experiment). This is interpreted by the spillover of dissociated H-2 (or D-2) onto the silica-alumina, with spillover hydrogen forming Bronsted sites and spillover deuterium exchanging with OH.