The pore size of mesoporous silica is precisely controlled by repeated surface treatment with tetramethyl orthosilicate (TMOS) and water. This surface treatment produces a silica monolayer by reaction between surface hydroxy groups and TMOS, and successive water treatment regenerates the surface silanol groups ready for further TMOS modification. This treatment method reduces the pore size of SBA-15 mesoporous silica incrementally by ca. 0.5 nm per treatment step, while preserving the original hexagonal pore structure.