Aging experiments on microporous sol-gel-derived nonsupported Si02 membranes were performed. Microstructure characterization was performed using nitrogen physisorption. It is found that both chemical aging and thermal aging result in densification of the microstructure, without pore growth. The influence of aging on supported SiO2-modified membranes was investigated using gas permeation and separation experiments. As for the nonsupported materials, some densification takes place. This leads to lower permeation rates, but a strong positive effect was observed on the separation properties. This might be attributed to a decrease of the pore size. Separation factors ranging from 50 to 125 have been measured for H2/CH4 at temperatures in the order of 250°C.