Abstract A detailed study of the changes that take place in the organic solvent extractable organic matter during the early stages of thermal maturation has not been published to date. The present work describes several trends occuring with increasing diagenesis, observed in the chloroform extractable organic matter of a series of coals of Tertiary age from the Mahakam delta (Indonesia), with special consideration given to the lignite and sub-bituminous stages. Wide variations in the values of the total chloroform extracts were observed in this work and the extraction yields reached a maximum at the beginning of the sub-bituminous stage. This maximum is discussed and two hypotheses are offered which may provide an explanation for the trend. These are the formation of extractable compounds by defunctionalisation of kerogen during the lignite stage, followed by a defunctionalisation of compounds in the chloroform extract itself during the sub-bituminous stage, and/or a migration from deeper horizons.