The stability of MCM-48 was investigated as a function of pH. The material was treated with aqueous solutions of varying pH and the stability was determined by comparison with the untreated MCM-48. In acidic conditions of pH < 5.4, the MCM-48 structure was stable and the BET surface area remained high (>1000 m(2) g(-1)), while the mean pore diameter decreased by approximately 20%. In neutral conditions, this phase was completely removed, characterised by the absence of an X-ray diffraction pattern and a dramatic reduction in surface area to 450 m(2) g(-1) at pH 6.9. Treatment at pH 9.1 caused a transformation to a poorly ordered material, while further increased basic conditions caused significant structural collapse. (C) 2003 Elsevier Inc. All rights reserved.