The chemical and physical properties of two base-catalyzed silica gels systems were characterized. The sol-gel transition was studied by titration, rheometry, and NMR. Pyrolysis behavior was monitored by TGA and mass spectroscopy. Many characteristics were strikingly similar for the two base-catalyzed systems. However ammonium hydroxide additions resulted in heterogeneities at lower concentrations than ammonium carbonate. Ammonium carbonate enhanced oxidation on pyrolysis.