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Monolithic and transparent silica aerogels were synthesized by two step sol-gel method using TEOS (tetraethylorthosilicate) as precursor, HCl and NH4OH as catalysts and ethanol as solvent. The alcogels were dried using supercritical CO2 extraction method. The effects of several sol-gel parameters including TEOS concentration, gelation and aging temperature on both transparency and thermal conductivity were investigated. First, TEOS concentration, which was found to have a significant effect on optical properties, was varied from 20 to 70 % by weight and the sol having 60 % TEOS was found to be the most transparent one. The alcogels were aged in 50% water-ethanol solutions at 298, 323, 343 K among them 323 K aged sample was the best in terms of transparency. After these parameters were optimized for maximum transparency and minimum thermal conductivity, the effect of supercritical CO2 extraction temperature, pressure and duration on pore size distributions and transparency were studied to relate these two properties. The extraction temperature was varied from 308 to 358 K and the pressure was varied from 80 to 300 MPa. It was observed that higher temperatures lead to better properties in terms of transparency. The drying duration was related to the drying conditions and the alcogel size. After drying step the aerogels were heat treated which increased transparency of the monolith beyond a certain temperature. For the transmittance measurements of the monoliths a UV-VIS spectrophotometer was used. Moreover the refractive indexes were determined by a laser method. The thermal insulation property was characterized via hot-wire method. Long term stability against high temperature tests were performed by subjecting the monoliths up to 1073 K for extended periods and also by subjecting the monoliths to rapid heating and cooling cycles.