Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 39 was obtained by hydrothermal synthesis with a gel composition of 6 SiO2:0.1 Al2O3:1 hexadecyltrimethylammonium chloride:0.25 dodecyltrimethylammonium bromide:0.25 tetrapropylammonium bromide:0.15 (NH4)(2)0:1.5 Na2O:300 H2O. The MCM-41 sample incorporating aluminum (AlMCM-41) was calcined in O-2 flow at 813 K and subsequently ion exchanged with Na+, K+, Ca2+, and Y3+. Th, ion exchange levels have been increased as high as 0.41 Na/Al, 0.36 K/A1, 0.37 Ca/A1 and 0.39 Y/A1, respectively. Hydrothermal stability of the ion exchanged A1MCM-41 materials has been investigated with X-ray powder diffraction pattern, BET specific surface area, and magic-angle spinning Al-27 NMR spectroscopy, after the samples were heated for 2 h at various temperatures in O-2 flow saturated with water vapor at room temperature. The hydrothermal stability of the AlMCM-41 has been found to extend to the following temperatures depending on ion exchange : Y3+ (1170 K) approximate to Ca2+ > Na+ (1070 K) approximate to as calcined AlMCM-41 > pure-silica MCM-41 (980 K).