The aim of this study is the determination of the gas adsorption capacity of Carboniferous coals in the Zonguldak basin, NW Turkey and its controlling factors. Investigated coals are typical humic coals rich in vitrinite and are high to medium volatile bituminous in rank. In order to determine gas adsorption capacity of coals, carbon dioxide gas adsorption isotherms are obtained by gas flow controlled and volumetric adsorption methods. BET, Langmuir and D-R equations were used to interpret the adsorption isotherms. Respective values of the BET, Langmuir and Dubinin-Radushkevich (D-R) adsorbed carbon dioxide volume vary between 2-13, 5-36 and 10-48 cm(3)/g at STP. Langmuir monolayer gas volume is considered as the gas adsorption capacity of the coals. The variations in this capacity have been examined by correlation with different coal properties determined with the help of geochemical, organic geochemical, and organic petrographical methods, Additionally, micropore volume which is determined using the D-R equation is correlated with coal rank and maceral type, and ash content. (C) 2000 Elsevier Science Ltd. All rights reserved.