Electron spin resonance has been employed to study the amount of Cu(II) ion-exchanged into mesoporous aluminosilicate molecular sieve AIMCM-41 containing variable amounts of aluminum in the tetrahedral framework. The results are compared with Y zeolite, alumina and MCM-41 with octahedral extra-framework aluminum. The Cu(II) ion change capacity per unit aluminum of AIMCM-41 is higher than that of Y zeolite, alumina and MCM-41 with octahedral aluminum, and the overall Cu(II) ion exchange capacity increases with increasing tetrahedral aluminum content in AIMCM-41. This suggests that the incorporated aluminum is isolated in the framework and serves as ion exchange sites as in zeolites. The maximum Cu(II) ion exchange capacity corresponds to about 44% of the aluminum sites and is achieved by a single ion exchange in 1 h at 95ºC. Apparently about half of the aluminum sites are not accessible to promote ion exchange. The aluminum sites active for ion exchange are postulated to be located on the internal surface of the hexagonal tubular walls of AIMCM-41.