Aluminosilicate MCM-41 catalysts derived from pure silica materials of varying particle size were prepared via postsynthesis grafting routes. The effect of particle size (which does not change on Al insertion) on the preparation and properties of the Al-grafted materials was studied using a variety of techniques. Particle size has no effect on the uptake and retention of Al (similar Si/Al molar ratios are obtained) but influences the distribution of Al, with larger particles having a higher concentration of surface Al. However, XPS and Al-27 MAS NMR studies show that particle size does not radically alter the way in which the surface or bulk Al interacts with the silica framework. Physical characterisation (XRD and N-2 sorption studies) indicates that particle size plays a key role in determining whether or not structural integrity is retained after Al insertion; large-particle pure silica materials, which possess long-range ordering, are more likely to retain their structural ordering and textural properties after Al insertion. The acid content land therefore proportion of Al sites giving rise to acid sites) is higher in small-particle samples than in large-particle samples. However, particle size has no effect on acid strength. The catalytic activity (for the cracking of cumene) is much higher for small-particle samples; this is presumably due to the higher efficiency with which reactant and product molecules are able to access or exit the shorter pores in small particles as opposed to the much longer tone-dimensional) pores in the larger particles.