A series of CoMo/-Al2O3 catalysts were prepared with an addition of 0.0, 0.5, 1.0, or 1.5% phosphorus pentaoxide to investigate the influence of phosphorus addition on the simultaneous hydrodesulphurization (HDS) of refractory sulphur compounds. Two sets of HDS experiments were carried out using sulphur bearing model compounds: (i) DBT (dibenzothiophene) with 4-MDBT (4-methyl dibenzothiophene) and (ii) DBT (dibenzothiophene) with 4,6-DMDBT (4,6-dimethyl dibenzothiophene). The Langmuir-Hinshelwood mechanism based kinetics model was developed from the experimental conversion and product distribution data for simultaneous HDS of the two sulphur compounds via direct desulphurization (DDS) and hydrogenation (HYD) routes. The kinetic model fits the experimental data quite adequately for all the species. The HDS rate for DBT was found to be approximately 2 and 7 times higher than that estimated for 4-MDBT and 4,6-DMDBT, respectively. Enhancement of HDS rate by P addition was observed up to 0.01g/g (1.0wt%) P2O5 while a higher amount of P is not advantageous. A marginal preference towards HYD pathway was observed for the HDS of 4,6-DMDBT while the DDS route was predominant during the HDS of DBT and 4-MDBT.The trends in values of the rate constants estimated by the Langmuir-Hinshelwood kinetic model compared well with pseudo-first rate constants.