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 four European research laboratories. in a first round of measurements, excess sorption isotherms were determined on Filtrasorb 400 (F400) activated carbon at 318 K using the manometric (TU Delft and RWTH Aachen University) and the gravimetric (FP Mons and INERIS) method up to 16 MPa. The study shows that CO2 sorption in the supercritical range can be determined accurately with both gravimetric and manometric equipment but requires thorough optimization of instrumentation and measuring as well as proper sample preparation procedures. For the characterization of the activated carbon F400, which we used as benchmark, we have determined a surface area of 1063 m(2) g(-1), and Dubinin-Radushkevich (DR) micropore volume of 0.51 cm(3) g(-1). Additionally, we analysed the elementary near-surface composition by energy dispersive X-ray spectroscopy (EDX). To characterise the bulk composition of the F400 activated carbon, a proximate and ultimate analysis was performed.The observed excess sorption maxima around 5 MPa have values around 8.0 mol kg(-1), which are consistently higher (by upto 0.8 mol kg(-1)) than literature data. (C) 2009 Elsevier Ltd. All rights reserved.