Utilizing the flexibility of the structure of MCM-41 type mesoporous metal silicates pore size modification was successfully performed for some MCM-41 samples including materials which contain Al or Ti heteroatoms in the wall or organic modifiers covalently bounded to the wall. Changing, i.e. expanding or narrowing the pore diameter was made by post-synthesis treatments. The products of these procedures were characterized by XRD, BET, IR and Si-29 MAS NMR techniques. For the expanded samples, a well characteristic thinning while for narrowed ones a definite thickening of pore walls, were established which were also accompanied by changes in the BET areas. The modified MCM-41 specimens retained their semi-crystalline character and the modification did not result in an increase of the concentration of structural imperfections.