Prior studies with kaolin particles have shown the acceptability of Urick’s theory of the dependence of sound velocity on the volume fraction of the suspensions (1, 2). So to calculate the particle compressibility from his equation, we had to know both the volume fractions and the densities of the particles. Therefore the possibilities of the determination of these quantities were investigated for gel-like Stober silica in water by measuring the sedimentation coefficients of the particles and the densities of their suspensions. Large differences have been found between the "dried" and "wetted" densities of silica, on the one hand, and between those of their volume fractions, on the other. These differences can be interpreted by looking at the 64% water content of the particles. Thus an effective density approach is proposed for the application of Urick’s equation, taking into consideration the amount of tapped fluid associated with gel-like or colloid size particles. The measured dependence of sound velocity (at 10 MHz) on the wetted volume fractions follows the prediction of Urick’s theory surprisingly well in the investigated 0-0.35 volume fraction range, giving a 2.8 x 10(-10) Pa-1 compressibility for the silica particles with water inside, while particle compressibility calculations using the dried density of the particle give a physically unrealistic result.