Cation exchange capacity (CEC) is among most important properties of sedimentary rocks, broadly applied in various means of geosciences research and industry. The organic matter (OM) in ancient black shales is thought to be a negligible source of cation exchange capacity (CEC), due to the loss of polar functional groups from extensive diagenetic transformations that occur during burial. OM in modern soils and sediments contains weakly bound hydrogen on carboxyl and phenolic hydroxyl groups, providing negatively charged functional groups that facilitate CEC on the order of hundreds of cmol(+)/kg. Kerogen in ancient sediments may either retain a portion of polar oxygen groups or these functional groups can be (re)gained upon drying, revealing an overlooked source of charge in black shales. Analyzing an extensive series of shales from the Baltic Basin (Poland) and Marcellus Shale (USA) with varying OM content and diagenesis we found that CEC, measured using Hexamminecobalt(III), of heated samples (>= 200 degrees C) is up to ten times greater than CEC measured on air-dry samples. Moreover, CEC measured on the heated samples is greater than theoretical CEC estimated from clay minerals composition. The excess CEC correlates with the content of oxygen-rich groups determined with OM pyrolysis and infrared spectroscopy. Carboxyl groups formed in OM due to thermal oxidation at temperatures ranging from 200 degrees C to 310 degrees C, in the presence of oxygen and under vacuum, are responsible for excess CEC. Our results reveal that kerogen in black shales is not chemically inert in the case of cation exchange and the OM can provide a considerable portion of the apparent CEC measured in bulk rock samples. (C) 2016 Elsevier B.V. All rights reserved.