Abstract: A process for isomerising a feed comprising aromatic compounds containing 8 carbon atoms is carried out in two steps: an isomerisation step and a dehydrogenation step. In FIG. 1, the feed to be treated is introduced into isomerisation zone R1 via line 1. Substantially pure hydrogen is introduced into line 1 via line 12 and recycled hydrogen is introduced into line 1 via line 13. Hydrogen which circulates in line 13 is purged via line 15. The effluent from isomerisation zone R1 is sent to a separation zone S1 via line 2. In S1, hydrogen contained in the effluent is isolated and recycled to the inlet to isomerisation zone R1 via line 13, the remaining effluent being evacuated from this separation zone S1 via line 3. The fluid contained in line 3 is heated in an oven F1 then evacuated therefrom via line 4. The effluent leaving the oven is enriched in recycled hydrogen via line 14 then it is introduced into dehydrogenation zone R2. The effluent from zone R2 is sent via line 5 to separation zone S2.

Abstract: For isomerising aromatic compounds containing eight carbon atoms in the presence of hydrogen, a mixture containing compounds with a boiling point of about 80° C. to 135° C. comprising at least one paraffin containing eight carbon atoms, at least one naphthene containing eight carbon atoms, at least benzene and at least toluene is added to the feed before introducing it into the isomerisation reactor R via line 1. These additions are made by a recycle via line 6 or by adding fresh compounds via line 11, or be a combination of fresh and recycled compounds via lines 6 and 11. Hydrogen is added via line 15.