Mesoporous Na-GaMCM-48 molecular sieves having a silicon-to-gallium (molar) ratio of 30-90 were synthesized hydrothermally, and the structure; coordination geometry of the gallium, acidic properties, and catalytic activity were systematically investigated using a number of analytical and spectroscopic techniques. XRD, TEM and N-2 sorption investigations indicate an MCM-48 structure with highly ordered mesoporosity. Ga-17 MAS-NMR studies reveal that gallium substitutes isomorphously in the mesoporous silicate framework of MCM-48. The NH3-TPD profiles suggest the presence of high concentrations of moderate-to-strong Bronsted acid sites in H-GaMCM-48. The catalytic performance of this protonated catalyst was evaluated for the t-butylation of phenol. The results indicate that the H-GaMCM-48 catalysts are highly active for the chosen reaction, and show much higher substrate conversion than many other catalyst systems. However, compared to the analogous H-GaMCM-41, the H-GaMCM-48 shows a slight decrease in p-t-butyl phenol selectivity owing to the formation of 2,4-di-t-butyl phenol. On the other hand, the deactivation is very minimal on account of the three-dimensional pore system of the MCM-48 structure compared to the one-dimensional pore opening of the MCM-41 structure. (C) 2004 Elsevier B.V. All rights reserved.