The experimental results on the hydrophobic and physical properties of tetraethoxysilane (TEOS) based silica aerogels by incorporating phenyltriethoxysilane (PTES) as a synthesis component, are reported. The molar ratio of tetraethoxysilane (TEOS), ethanol (EtOH), water (0.001 M oxalic acid catalyst) was kept constant at 1: 5: 7 respectively while the molar ratio of PTES/TEOS (M) was varied from 0 to 0.7. After gelation, the alcogels were dried supercritically by the high temperature solvent extraction. For lower M values (<0.1), transparent and monolithic aerogels but for higher M values (>0.5) opaque and cracked aerogels were obtained. For M values in between 0.1 and 0.5, monolithic aerogels with decreasing optical transmission have been obtained. The hydrophobicity of the aerogels was tested by measuring the percentage of water uptake by the aerogels when exposed to 95% humidity at 40degreesC for 24 h and also by measuring the contact angle. The contact angle varied from 120 to 130degrees for M values from 0.1 to 0.5, respectively. It was found that as the M value increased, the hydrophobicity of the aerogels increased but the optical transmission decreased from 60% to 5% in the visible range. The thermal conductivity and the specific heat of the aerogels found to decrease with the increase in M values. In order to determine the thermal stability in terms of retention of hydrophobicity of the aerogels, they were heat treated in air in the temperature range of 25-600degreesC. The hydrophobic aerogels are thermally stable upto a temperature of 520degreesC, which is the highest value ever reported, and above this temperature the aerogels become hydrophilic. The chemical bonds, responsible for the hydrophobic nature of the aerogels, have been identified by Fourier transform infrared spectroscopy (FTIR). The aerogels have been characterized by density, optical transmission, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential thermal analysis ( DTA). (C) 2003 Kluwer Academic Publishers.