The invention concerns a process for reducing the benzene content of petrol fractions, in which hydrogenation is carried out on a feed with the following composition by weight: 40% to 80% of paraffins, 0.5% to 7% of cyclic hydrocarbons and 6% to 45% of aromatics, and with a maximum distillation temperature of between 70.degree. C. and 90.degree. C., followed by isomerisation of the effluent from the hydrogenation step, mixing said feed and/or said effluent with a C.sub.5 -C.sub.6 cut; said process being characterised in that an isomerisation catalyst is used which contains chlorine and at least one group VIII metal deposited on a support composed of a mixture of specific proportions of eta alumina and gamma alumina.