Shape-selective isopropylation of naphthalene over a hydrogen-mordenite (HM) catalyst is an effective route for the production of 2,6-diisopropylnaphthalene (2,6-DIPN). Our previous work on naphthalene isopropylation has shown that 2,6-DIPN selectivity is substantially increased by HM dealumination. An assessment of 2,6-DIPN reactivity on the catalyst would be useful in understanding factors that control product selectivity. We measured 2,6-DlPN reactivity on three HM catalysts (SiO2/Al2O3 ratio 14, 38 and 74), under propylene pressure, in batch reactor tests at 200 degrees C. The propylene pressure was varied to give propylene to 2,6-DIPN mole ratios of 4.0, 1.0 and 0.5. On HM14 and HM38 catalysts, 2,6-DIPN conversions were 12-27%, mostly to higher alkylates. Some isomerization of 2,6-DIPN occurred at the lowest propylene pressure, but was suppressed by increasing the propylene pressure. 2,6-DIPN is nearly unreactive on dealuminated mordenite (HM74). These results reveal that the high selectivity toward 2,6-DIPN in the isopropylation of naphthalene is partly attributable to the low reactivity of 2,6-DIPN on dealuminated mordenites.