The properties of organically modified mesoporous silicates were shown to be influenced by the amount of modifier used in the synthesis reagents. Synthesis at 100 degrees C gave MCM-48 without the organic modifier while the addition of 10 mol% 2-cyanoethyltriethoxysilane modifier produced MCM-41 at the same temperature. Increased temperatures are required in both preparations to achieve an optimum amount of MCM-48, kamely 125 degrees C without modifier and 150 degrees C with modifier while a disordered phase (resulting from the decomposition of MCM-50 during calcination) was formed at higher temperatures. The influence of synthesis time showed that the formation of MCM-48 was preceded by the appearance and subsequent collapse of a mixture of MCM-41 and MCM-50 mesophases. The MCM-48 phase then developed over time reaching an optimum after 20 h synthesis at 150 degrees C. Thereafter MCM-48 was replaced by a disordered material (decomposed MCM-50) and was accompanied by an increase in mean pore diameter and a large decrease in surface area. After 60-70 h, the MCM-48 phase was completely transformed to the disordered phase. These findings are rationalised on the basis that the silica charge density changes during synthesis, leading to rearrangements in mesopore phase and increased pore diameters to maintain charge matching. (c) 2006 Elsevier B.V. All rights reserved.