The title reaction was investigated over three MOR zeolites with Si/Al ratios of 10, 60 and 80. Surface acidity was evaluated by adsorption microcalorimetry and FTIR analysis, using pyridine as probe molecule. Catalytic tests were carried out at 623 K and 40 bar in liquid-phase, with decalin as solvent, in a flow reactor. The formation of isopropylnaphthalenes (mainly monoisopropylnaphthalenes) was accompanied by fast oligomerisation/cracking of propene resulting from isopropanol dehydration and trapping of carbonaceous compounds ("coke") in the zeolite pores. This caused a quasi-immediate blockage of the access to the micropores, suggesting that reactions occur on or near the outer surface of the crystallites. In agreement with this proposal, the activities of the three MOR samples, which have very different acidic properties but similar textural properties, were identical. A reaction scheme involving as a main step transalkylation between naphthalene molecules and the isopropyl aromatic molecules trapped in the MOR channels is proposed to explain the formation of isopropylnaphthalenes over the aged MOR samples. In agreement, isopropylnaphthalenes formation was found to remain significant for more than 1 h after substituting a decalin-naphthalene mixture for the normal feed.