RF aerogels were prepared by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The aerogels were characterized by nitrogen adsorption and density measurements. A small-angle X-ray scattering (SAXS) technique was applied to the gelation process of RF hydrogels. The structure formation of the hydrogels during the sol-gel transition was revealed by applying Guinier and power-law equations to the SAXS data. At the initial stage of the synthesis of RF hydrogels, small clusters of ca 2 nm consisting of branched polymeric species formed, showing a mass fractal dimension. Then the clusters aggregated and formed particles of ca 3-6 nm. These particles showed a surface fractal dimension. The hydrogel structure was fixed by gelation and the particles grew to ca 4-7 nm. Finally their surface became smooth by aging. The influence of the amount of resorcinol, basic catalyst, and water used in the polycondensation on the porous structures of the aerogels was explained by the structure formation model proposed,