Resorcinol-formaldehyde (RF ) aerogels were synthesized via the sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and followed by supercritical drying with carbon dioxide. Mesoporous carbon aerogels were then obtained by pyrolyzing the RF aerogels in an inert atmosphere. The control of mesoporous structure of the aerogels was studied by changing the amount of resorcinol (R), formaldehyde ( F ), distilled water ( W ) and sodium carbonate (basic catalyst) (C ) used in the polycondensation. As a result of characterization by nitrogen adsorption, the mesopore radius of the RF aerogel was controlled in the range of 2.5-9.2 nm by changing the mole ratio of resorcinol to sodium carbonate (R/C ) and the ratio of resorcinol to water used as diluent (R/W ). Although the aerogels shrank by 1-4 nm during pyrolysis, the shape of pore size distribution of the RF aerogel was kept. It was found that the mesopore radius of carbon aerogel ranged from 2.0 to 6.1 nm. As the pyrolysis temperature increased, the peak radius of pore size distribution was kept the same despite the fact that the pore volume decreased because of shrinkage. Adsorption isotherms of ethane and ethylene were measured on the aerogels prepared. As the pyrolysis temperature increased, the amounts of ethane adsorbed became larger than those of ethylene adsorbed on the aerogels. The aerogels pyrolyzed at 1000Β°C had the same adsorption characteristics of ethane and ethylene as the activated carbons did.