Hydrodesulfurization of thiophene in n-heptane, using a mixture of cobalt and molybdenum oxides as catalyst, has been studied in a fixed bed integral reactor, 200 mm long, 30 mm outer diameter and 20 mm inner diameter. It has been shown that, under experimental conditions employed in this study, neither external nor internal diffusion had been effective in the process and isothermal conditions prevailed in both gas and solid phases. A three step mechanism has been proposed for the hydrodesulfurization of thiophene. According to such a mechanism the diluent (n-heptane) acts as an inhibitor by occupying free active sites. In further steps, thiophene is adsorbed on the catalyst surface and then undergoes reaction with hydrogen gas. Based on this scheme a rate model has been derived and verified by experimental results.