Alumina-supported Ni-promoted W oxides, carbides and nitrides were presulfided and employed for reducing pollutants and increasing the quality of an extra-heavy crude oil during catalytic upgrading reactions. These materials were prepared by temperature programmed reaction using reactive gases and varying the (Ni/(Ni + W)) atomic ratio from 0.00 to 1.00. Catalysts were characterized by XRD, SEM, HRTEM and N-2 physisorption. In order to study the effect of the atomic ratio in the conversion of the model molecule thiophene, room pressure HDS tests were used. The catalysts with best performances during thiophene HDS were tested in the upgrading of a Venezuelan extra-heavy crude oil characterized by having high asphaltenes, S and N contents, affecting competitiveness in the global market. These results were compared with a commercial NiMo catalyst. API gravity of crude oil, CHNS elemental analysis, C-13- and H-1 NMR of crude oil and asphaltenic fraction, in addition to flocculation threshold of asphaltenes, were studied in order to verify variations in physicochemical properties of oil due to catalytic upgrading and to seek evidence of pollutants reduction and improvement of quality during this process. NiW catalysts with atomic ratio (Ni/(Ni + W) = 0.50 showed remarkable performance during thiophene HDS and heavy oil hydrotreatment, improving API gravity and reducing S content, modifying chemical nature of crude oil and asphaltenes, as it was revealed by results of elemental analysis, H/C ratio, flocculation threshold, C-aro/C-ali and H-aro/H-ali ratios. However, no significant variations were found in N contents of crudes and asphaltenes revealing poor HDN performance, apparently due to Na traces from the W precursor remaining in the final NiW catalysts.