The structure and energy of the tert-butyl cation on zeolite Y were calculated at ONIOM(MP2(FULL)/631G(d,p):MNDO) level. The results indicated that the tert-butyl cation is a minimum and lies between 40 and 51 kJ mol(-1) above in energy to the tert-butoxide, depending on the level of calculation. Both species are stabilized through hydrogen bonding interactions with the framework oxygen atoms. Experimental data of nucleophilic substitution of tert-butylchloride and bromide over NaY impregnated with NaCl or NaBr give additional support for the formation of the tert-butyl cation as a discrete intermediate on zeolite Y, in agreement with the calculations. (C) 2009 Published by Elsevier B. V.