Abstract Detailed analyses of RICO (ruthenium ion catalyzed oxidation) products of bituminous and brown coals were conducted by using FD/MS and 13C-NMR. These analyses indicate that the aqueous phase fraction (water-soluble products) from RICO of brown coals consists of aliphatic dicarboxylic acids and aromatic polycarboxylic acids with longer alkyl side chains. The presence of benzene tetra-, penta-, or hexacarboxylic acid in the RICO products of brown coals might indicate that brown coals contain relatively large aromatic clusters with three or more rings. On the other hand, a similar fraction from RICO of bituminous coals seems to consist of mainly non-substituted aromatic polycarboxylic acids along with a small amount of aliphatic dicarboxylic acids. These differences could be explained based on the structural features of the sample coals. Bituminous coals contain relatively large condensed aromatic rings with fewer alkyl side chains or bridges, these easily being converted to aromatic polycarboxylic acids, while brown coals are comprised of smaller aromatic clusters with larger numbers of alkyl side chains or bridges, a portion of the aliphatic functional groups being converted to free acids with the remainder attached to the aromatic rings (i.e. production of ring-alkylated benzene polycarboxylic acids). Analysis of organic solubles from brown coals by 13C-NMR and FD/MS indicates that this fraction contains a large amount of aliphatic carboxylic acids with longer alkyl groups up to carbon number 38, suggesting that brown coals contain longer alkyl side chains (up to 38) on aromatic rings.