A series of CoMo/gamma-Al2O3 catalysts modified with P2O5 were prepared. The phosphorus concentration varied from 0.0 to 1.0 wt.% P2O5. The catalysts prepared were evaluated in a batch autoclave reactor to investigate the effect of P2O5 on the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4-methyl dibenzothiophene (4-MDBT). The phenomenological based kinetics models are developed based on the experimental conversion and selectivity data. The analysis of the developed model suggests that a Langmuir-Hinshelwood mechanism fits the experimental data adequately. The rate constants for the formation of BP are 6-8 times higher than the rate constants for the formation of CHB. Similarly, the rate constants for the formation of MBP are 3-5 times higher than the rate constants of MCHB formation. These observations indicate that the HDS of the model compounds through the DDS route is several times faster than the HDS through the HYD route. Furthermore, the rate constant for the formation of BP and CHB is about two times higher than the respective rate constant for the formation of MBP and MCHB. The addition of P2O5 favored the DDS pathway over the HYD pathway for both DBT and 4-MDBT. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.