Isomorphously substituted GaMCM-41 molecular sieves with different Si:Ga ratio were synthesized and characterized systematically by various analytical and spectroscopic techniques. Temperature-programmed desorption of ammonia was used to identify the nature of acidic sites. For the first time, the vapor-phase tertiary butylation of phenol reaction was carried out over protonated GaMCM-41. The catalysts showed higher (phenol) conversion and good (para-tertiary butyl phenol) selectivity over the Al- and Fe-analogs of MCM-41. Further, the gallosilicates showed good stability both before and after the reaction compared to the aluminosilicates or ferrisilicates.