The preparation of highly dispersed gold nanoparticles in mesoporous silica SBA-15 is monitored by in situ XAS measurement. The silanation of intrachannel surface with TPTAC can generate ion-exchange sites for anionic Au(OH)(n)Cl-4-n(-) (with n = 0-4) complex ions and help to produce uniformly dispersed gold nanoparticles in the host channels after further reduction. The average size of metal nanoparticles is dependent on the pH value of gold precursor solution. (C) 2004 Elsevier B.V. All rights reserved.