A CFD benchmark was performed within the Hylndoor project, to study hydrogen release and dispersion in a confined space with natural ventilation and one vent. Three experiments, performed earlier by CEA at their GAMELAN 1 m(3) facility, were considered for this benchmark. In all three tests helium (instead of hydrogen for safety reasons) was released vertically upwards at 60 NL/min from a 20 mm orifice near the centre of the enclosure. A different vent size was used for each test. Three HyIndoor partners (JRC, NCSRD and UU) participated in the benchmark, with three different CFD codes, (ANSYS Fluent, ADREA-HF and ANSYS CFX) and three different turbulence models respectively (transitional SST, standard k-epsilon, dynamic Smagorinsld LES). In general, good agreement was found between predicted and measured helium concentrations. However, in the case of the vent with the smallest vertical extension (vent c) all predictions overestimate the concentration at the lower part of the enclosure at steady state. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.