Several mild hydrocracking 3 wt.-% Co-10 wt.-% Mo/B-Al2O3 catalysts have been prepared by adding H3BO3 into a boehmite dough during kneading + extrusion, drying and calcination prior to impregnation. HDS, HDN and cracking were monitored in the hydrotreatment of a VGO feedstock and the optimum B content was estimated around 3 wt.-%. The effects of boron on these catalysts will be discussed here with a particular focus on the effect of boron during impregnation. Using a simple hydrothermal treatment method by soxhlet, B can be extracted, with hot water, from catalysts and from supports. Simultaneously, on supports calcined at 600 degrees C, the formation of a boehmite phase was detected after extraction.