A new method for the synthesis of large mordenite crystals from heterogeneous aluminosilicate reaction mixtures is presented. The method involves heat treatment of porous silica gels in air at elevated temperatures prior to their use in mordenite synthesis. Heat treatment lowers the surface area and pore volume (porosity) of X-ray amorphous porous silica gel particles without substantially changing the size of the remaining pores. Thus, a decrease in accessibility of the internal surface area of silica to dissolution is realized. The result is nucleation of fewer mordenite crystals which grow larger. The method employed to grow large mordenite crystals also leads to the formation of other crystalline phases (quartz and/or zeolite P_c) coexisting with mordenite, the amounts of which increase with increasing heat treatment temperature of silica. For the investigated composition only large-pore (140 and 150 Å) silica gels resulted in the synthesis of large mordenite crystals (up to 175 µm) when they were heated to 800–900 °C prior to use. The use of heat-treated, medium-pore (60 Å) silica gel in zeolite synthesis resulted in growth of substantially smaller mordenite crystals with size up to 80 µm. Small-pore (22 and 25 Å) silica gels could not be used to grow large mordenite crystals by heating them before use in syntheses. Upon heating at elevated temperatures prior to use, these silica gels resulted in synthesis of predominantly quartz and no significant size increase of mordenite crystals was observed at any heattreatment temperature. It is also shown that the combination of heat and mechanical (grinding) treatments of porous silica allows control of the crystallization and the size of mordenite crystals.