MCM-41 mesoporous molecular sieves were synthesized at low temperature (343 K ) in the presence of cetyltrimethylammonium bromide (CTMABr) and subjected to postsynthesis hydrothermal treatment at elevated temperatures (413 K and 423 K ). The hydrothermal stability of good quality large pore MCM-41 obtained by restructuring at 423 K was found to be comparable with that for the most stable MCM-41 materials reported in the literature. However, restructuring in the mother liquor at 413 K did not lead to good quality large pore MCM-41 materials. Adsorption measurements revealed limited pore size enlargement, accompanied by the development of microporosity and appearance of constrictions in the mesoporous structure. The similarity of adsorption isotherms for some of these materials and those of the ordered mesoporous molecular sieve SBA-2 with a three-dimensional cage-like pore structure suggests the presence of disordered arrays of mesoporous cages with no well-defined shape, size and connectivity. It was also shown that the hydrothermal treatment in water at 423 K led to initial improvement of the ordering, but the pore size increase was relatively small (ca. 10%). Extended treatments afforded materials with significantly enlarged unit cells, but with lower degree of pore structure ordering. Attempts were made to compare different approaches to the synthesis of good quality large unit cell MCM-41 materials templated by CTMABr.