The influence of phosphate (P) on the structure of alumina-supported Co—Mo catalysts was studied by means of temperature-programmed reduction (TPR). Conclusions deduced from these TPR measurements were supported by Fourier transform infrared (FT-IR) and Raman measurements. Phosphorus added by sequential impregnation or coimpregnation had a great influence on both Co and Mo surface structure. Addition of P to an alumina support resulted in the formation of a well dispersed AlPO4 surface layer. Next to the phases already reported on Co/Al2O3 catalyst, cobalt can be present in interaction with the AlPO4 species in Co—P/Al2O3 catalysts. Coimpregnation of Co and P increased the dispersion of the cobalt species. The influence of P on the distribution of the Mo species depends on the P content and the sequence of impregnation. On Mo—P catalysts, prepared by coimpregnation, the reducibility of a part of the Mo species decreased with increasing P content due to the formation of Mo—O—P species. On the catalysts prepared by sequential impregnation of P and Mo a part of the Mo was present in MoO3 crystallites, the other part was very well distributed over the AlPO4 surface layer. These results suggest that at high P loadings, Mo can be present as a Mo-AlPO4 surface structure. Both sequential and coimpregnation of Co, Mo and P resulted in the formation of a Co—Mo—O—P phase, in which Co is in interaction with MoO3 and AlPO4 species. Due to the stronger polarization of the Co—O bond by P5+, the reducibility of the Co2+ -ions in the Co—Mo—O—P phase is decreased compared to the reducibility of these ions in the Co—Mo—O phase. At a P-to-metal ratio of = 1, the AlPO4 species were completely reduced to metalphosphides and alumina above 1000 K under TPR conditions. At higher P loadings, a part of the phosphates was reduced to P4. The reduction of AlPO4 is accelerated by both reduced Co and Mo species.