Nanoscale gold particles dispersed within pores of mesoporous silica were synthesized by soaking and thermal reduction of chloroauric acid (HAuCl4) in H-2 atmosphere at 973 K for 1 h. This new material was characterized by X-ray diffraction, transmission electron microscopy, and Brunauer-Emmett-Teller (BET) techniques. It is shown that gold nanoparticles are isolated from each other and uniformly dispersed inside the pores of silica, which were less than 4 nm in diameter. It is found that in optical absorption spectra, the Mie resonance frequency of gold particles shows a significant red-shift with decreasing Au particle size. This red-shift phenomenon is qualitatively explained in terms of the interface interaction between the pore walls of porous silica and Au particles within the pores. (C) 2000 Elsevier Science Ltd. All rights reserved.