Nano- and mesopores of bulk silica xerogels were studied by means of positron annihilation lifetime measurement as well as nitrogen physisorption. Effects of an alkaline catalyst concentration in the sol-gel process on the pore sizes and specific surface area were investigated. The pore size was estimated from the ortho-positronium lifetime. Remarkable structural changes were found around a relative alkaline concentration of 10(-4) (a molar ratio of sodium hydroxide to tetramethyl ortho-silicate). Above this particular catalyst concentration, the nanopore radius decreased from 0.45 to 0.35 urn, whereas the mesopore radius increased from I to 8 mu. The positron annihilation lifetime technique was effective in the evaluation of the nano- and mesoscopic structural changes of silica xerogel.