The successful synthesis of M41S materials,[1,2] a new family of novel silicates with uniform mesopores, opened a vast range of applications[3-7] in catalysis, sorption, molecular recognition, electronics, and photochemistry. One of the members of this family of materials is MCM-41, which contains a hexagonal array of mesopores with a pore diameter of 20 100,A depending on the conditions of its synthesis. It is postulated[2] that the inorganic walls around the mesopores of MCM-41 are constructed of disordered SiO4 tetrahedral units, as is also the case for amorphous silicates. NMR studies[2,8] reveal that MCM-41 contains many silanol groups both before and after the template molecules have been removed. These silanols may act as functional groups in chemical reactions or can be utilized to attach molecules having other functional groups. Although the structure and stability of the pores are well understood, little is known about the nature of the silanol groups in MCM-41. To shed some light on this, we monitored the IR absorptions of the silanol groups in template-free MCM-41 as a function of evacuation temperature before and after silylation by chlorotrimethylsilane (CTMS) and chlorotriphenylsilane (CTPS).