Low field NMR spin-lattice relaxation measurements of pore fluids contained in silica gels are employed as a pore structure probe to ascertain the changes that occur in a two-step acid/base-catalyzed silica gel during aging in its mother liquor. It has been shown that both a narrowing of pore size distribution and an increase in mean pore size is obtained for a gel aged at 303 K over a time period of several weeks. By increasing the aging temperature from 303 to 333 K, the time required for this same degree of pore size distribution narrowing is decreased by an order of magnitude. Increasing the gel solids content from 15 to 30 wt% SiO 2 results in smaller pore sizes and pore volumes (a factor of ∼ 2) but a similar pore size narrowing with comparable kinetics. In contrast, little change in pore structure is observed in a two-step acid-catalyzed gel aged in its mother liquor at 303 K for similar time periods.