An organic liquid species, tributyl phosphate (TBP), was immobilized in sol-gel derived silica matrix by direct mixing into a silica sol. These mixtures spontaneously solidified due to the polycondensation of silica sol. The obtained glassy materials were completely solid and optically transparent. TBP was stably immobilized as indicated by the absence of the leakage from the surface. Although the stable immobilization was macroscopically observed even at ca. 70% of the volume fraction of TBP in the glass sample, the results of the thermogravimetry-differential thermal analysis, Fourier transform infrared spectroscopy and immersion tests in water suggested that TBP is not chemically bound to the silica matrix but topologically entrapped in the cage of siloxane bonds. The relatively high Vickers hardness indicated that the network of siloxane bonds envelopes the immobilized TBP molecules. The results of wide angle X-ray scattering showed that the unit length of the siloxane bond does not significantly depend on the volume fraction of TBP. The expansion in the volume due to the incorporation of TBP occurred not uniformly but quite locally at the spots at which TBP molecules were embedded. The formation of the network of siloxane bonds is enhanced in the vicinity of the enveloped TBP molecules because of its Lewis basicity at the phosphate group. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.