Liquid phase isomerisation of dichlorobenzenes (dCBs), i.e. ortho-dichlorobenzene (o-dCB) and/or para-dichlorobenzene (p-dCB) to meta-dichlorobenzene (m-dCB) catalysed by solid acid catalysts could provide an alternative to the traditional process based on AICl(3)-type catalysts. The reaction was studied over a series of H-zeolites in a batch autoclave reactor at a temperature of 340 degreesC and a pressure of 40 bar, with pure reactants, i.e. without solvent. Under these conditions, H-ZSM-5 and mordenite exhibited the highest activity, while beta-zeolite, faujasite and ferrierite showed only low activity. These results are directly related to the pore architecture of the different zeolites, which governs deactivation by coke formation. The nature of the coke formed in the two most active zeolites H-ZSM-5 and mordenite has been analysed by different techniques.