Isopropylation of naphthalene was examined over two dealuminated H-mordenites (HM) having SiO2/Al2O3 molar ratio of 38 (HM38) and 74 (HM74). Experiments were done in batch reactors at 200 degrees C. With propylene as the alkylating agent, selectivity for beta-substitution of naphthalene increased when water was added, and reached a maximum with a water-to-catalyst mass ratio of 0.80 (ca. 45 mmol water/g-cat). Of particular importance was the increase in beta beta-selectivity to 2,6- and 2,7-diisopropylnaphthalene (2,6-DIPN and 2,7-DIPN) with added water. Without added water, beta beta-selectivities were 56 and 77% for HM38 and HM74, respectively. Selectivities to the highly desired 2,6-DIPN isomer were 37 and 54%, respectively. With added water, beta beta-selectivities increased to 90 and 95%, and 2,6-DIPN selectivities increased to 60 and 70% for HM38 and HM74, respectively. A large amount of added water always decreased the activity; however, mixed effects were observed for intermediate amounts of added water. Sorption of water on the catalyst surface seems to explain changes in isomer selectivity and catalytic activity. Coke deposits on the catalysts also decreased when water was added. Furthermore, it was shown that higher beta beta-selectivity could be obtained with isopropanol (i-PrOH) as the alkylating agent, rather than propylene, under the same conditions. Comparison experiments showed that this was due to water formed from i-PrOH.