The hydrocracking of vacuum gas oil (VGO) and a mixture of 90 wt% VGO and 10 wt% wax was investigated. The study provided insight into processing of naturally waxy petroleum, as well as refinery co-processing of Fischer-Tropsch products with petroleum. The investigation was performed using flow reactor system at kilogram per day scale. Hydrocracking was investigated over a sulfided NiMo/SiO2-Al2O3 catalyst in the temperature range 330-410 degrees C at 9.5 MPa and LHSV of 1.3 h(-1) with H-2-to-feed ratio of 620 normal m(3)/m(3). Atmospheric residue conversion of the VGO was 8-12% higher than conversion of VGO + wax at 330-380 degrees C, and 24% higher at 410 degrees C. Hydrodesulfurization conversion of the feed (3 wt% S) was less for VGO than for VGO + wax. Hydrodenitrogenation conversion of the feed (0.2 wt% N) was less for VGO than for VGO + wax. Directionally the observations could be explained in terms of competitive adsorption. However, the extent of the difference in atmospheric residue conversion was more than could be explained by competitive adsorption and VGO dilution by wax. It was postulated that changes in the bulk solubility characteristics of the liquid contributed to alter the relative concentration of species at the catalyst surface based on the change in the solubility parameter of the bulk liquid relative to the individual species. In this way it was possible to also account for the extent of the conversion difference observed during hydrocracking of VGO and VGO + wax at similar conditions.