Positron annihilation spectroscopy has been used to obtain information about the small-pore structure of the system xTiO 2 •(1 − x)SiO 2 (x = 10, 30 mol%). The pore radius probability distribution functions have been obtained from the ortho-positronium lifetime probability distribution functions, determined from lifetime spectra using the CONTIN (PALS-2) program. The linearity of the variation of S versus W has been used to check the similarity of (or difference between) the defect structures of the samples prepared under different experimental conditions.