In the field of hydrotreating (HDT) catalysis, density functional theory (DFT) calculations are of great help to explore new active phases on the basis of volcano curve relationships correlating HDT activities and the sulfur-metal (S-M) bond energy chemical descriptor, calculated in transition metal sulfides catalysts. In the present study, we synthesize Mo1-xWxS2 solid solutions supported on gamma-alumina. For non-promoted systems, the catalytic tests reveal that a continuous and linear evolution of the catalytic activity is obtained for solid solutions for x varying between 0 and 1. As expected from the calculated S-M bond energy values, no synergy effect is observed in that case. For Ni and Co promoted Mo1-xWxS2 active phases (ternary metal sulfides), the S-M bond energy values determined with an interpolation model of the binary sulfides predict that NiMo1-xWxS2 phases should be more active than NiMoS and NiWS ones. In contrast, CoMo1-xWxS2 phases are expected to develop weak synergetical effect with respect to CoMoS and CoWS ones. Experiments performed on Co and Ni promoted Mo1-xWxS2 active phases confirmed the DFT-volcano curve prediction and an increment of about 30% in HDS catalytic activity is obtained for NiMo0.5W0.5S catalysts in both model molecule conversion and gas oil treatment. (C) 2007 Elsevier B.V. All rights reserved.