In order to assess and improve the quality of high-pressure sorption isotherms of carbon dioxide (CO2) on coals, an inter-laboratory study (Round Robin) has been conducted among three European research laboratories. Excess sorption isotherms, determined in a first round of measurements, on Filtrasorb 400 (F400) activated carbon showed excellent agreement. In the second round of this study, excess sorption isotherms were determined on three coals at 318 K using the manometric (TU Delft, Netherlands and RWTH Aachen University, Germany) and the gravimetric (University Mons, Belgium) methods up to 16 MPa. The CO2 excess sorption isotherms for the three coal samples, a lignite, a bituminous coal and a semi-anthracite, exhibited maximum values of 1.77 +/- 0.07, 1.37 +/- 0.01 and 1.37 +/- 0.05 mol kg(-1) respectively. The pressure ranges for the observed maximum excess sorption capacities decreased with increasing maturities from 6.89 +/- 0.5 MPa for the lignite, to 6.68 +/- 0.4 MPa for the bituminous coal and to 5.89 +/- 0.6 MPa for the semi-anthracite. The results show that high-pressure CO2 excess sorption isotherms on natural coals in the supercritical range can be determined accurately with both gravimetric and manometric equipment. (C) 2010 Elsevier B.V. All rights reserved.