Highly transparent monolithic silica aerogels based on the TEOS precursor were prepared by the two-step (acid-base) sol-gel process. The hydrolysis and condensation reactions of tetraethoxysilane (TEOS) proceeded in methanol solvent with oxalic acid and NH4OH as the catalysts, respectively. The wet gels were supercritically dried using methanol. The aerogels were characterized by transmission electron microscopy (TEM) and measurements of optical transmission, bulk density, volume shrinkage, porosity and thermal conductivity. Monolithic silica aerogels with high optical transmission (similar to 93%), low density (similar to 0.055 g/cm(3)), low thermal conductivity (similar to 0.04 W/mK), and minimum volume shrinkage (similar to 10%), were obtained for the molar ratio of TEOS:MeOH:acidic H2O:basic H2O at 1:33:3.5:3.5 alongwith the oxalic acid and NH4OH concentrations at 0.001 M and 1 M, respectively.