The adsorption of water vapor and carbon dioxide at room temperature in relation to carboxylic functional groups on the surfaces of 20 different types of coal was examined. The carboxylic functional groups on the surface of coal may be considered the preferential sites of adsorption when compared with the other groups. The adsorption of water vapor and carbon dioxide was found to increase in proportion to the square root of the carboxyl group concentration, regardless of the partial pressure or the gas species. Assuming functional soup adsorption sites to be present on the surfaces of hydrophobic matrices and adsorption to occur in a liquid-like closed packed state, the adsorption on coal was determined based on the carboxyl soup concentration on the coal surface, the molecular cross section and the molecular weight of the adsorbate. The surface area did not depend on the type of adsorbate, as was observed for water vapor and carbon dioxide adsorption on coal. (C) 2001 Elsevier Science Ltd. All rights reserved.