Nickel or molybdenum were supported on niobium oxide, both pure and modified with phosphate. The catalysts and supports were sulphided at 673 K under a 15 mol% H2S in H-2 mixture. It was observed by X-ray photoelectron spectroscopy (XPS) that the sulphur uptake of the catalysts was much in excess to the amount necessary for complete sulphidation of nickel and molybdenum and occurred even with the non-impregnated supports. Simultaneously, a peak attributable to niobium in an oxidation state lower than +5 appeared in the XPS spectra. A good linear correlation was found between the atomic percentage of sulphur, above the one necessary to sulphide the supported metals, and the atomic percentage of niobium in the reduced state, showing that partial reduction-sulphidation of the support occurred during the sulphidation treatment. The slope of the best line through the experimental points was 1.99, indicating that a surface niobium sulphide with stoichiometry close to NbS2 was formed during the treatment. While the reduction-sulphidation of niobium was enhanced by the presence of supported nickel and especially molybdenum, it was inhibited by phosphate. The supports and catalysts were tested in the hydrocracking of cumene at 623 K and 2.7 MPa. When the materials were sulphided under 15% H2S/H-2 at 673 K, a linear correlation was observed between the activity and the extent of reduction-sulphidation of the supports, indicating that the surface niobium sulphides were principally responsible for their acidic activity. However, when the materials were sulphided at 623 K with the feed containing CS2 as a sulphiding agent, before the catalytic test, no such correlation was found and the activity in cumene hydrocracking was much smaller, showing that the lower temperature is insufficient for complete development of the support sulphidation, especially with the molybdenum catalysts. It was observed by XPS that, at 673 K, reduction of the molybdenum to the +4 state during sulphidation is nearly complete with an alumina support, but not with the niobia supports, which suggests the existence of a strong molybdena-niobia interaction.