Abstract Organic aerogels were synthesized via the sol–gel polycondensation of mixed cresol with formaldehyde in a slightly basic aqueous solution followed by supercritical drying with carbon dioxide. Carbon aerogels are generated by pyrolysis of organic aerogels in inert atmosphere at high temperature. Obvious chemical and physical changes can take place within aerogel microstructures during the pyrolysis process. IR combined with TGA, TEM, and nitrogen adsorption is employed to study these changes in detail. Appreciable transformation of aerogel structure occurs during 250–600°C, which results from the release of water,organic groups and simultaneous rearrangement of aromatic rings. A new band occurring at 874 cm −1 in IR spectra after 400°C is associated with the IR-active vibration states of graphitic structure. With the increase of pyrolysis temperature, the density, surface area and total volume of aerogels keep increasing; on the contrary, the pore size distribution becomes narrower and the pore size decreases. The carbon aerogel microspheres are smaller than their organic aerogel precursors as a result of shrinkage during pyrolysis, as seen from TEM results.