The paper describes an exercise on comparison of Computational Fluid Dynamics (CFD) models to predict deflagrations of a lean uniform hydrogen–air mixture and a mixture with hydrogen concentration gradient. The exercise was conducted within the work-package “Standard Benchmark Exercise Problem” of the EC funded Network of Excellence “Safety of Hydrogen as an Energy Carrier”, which seeks to provide necessary quality in the area of applied hydrogen safety simulations. The experiments on hydrogen–air mixture deflagrations in a closed 1.5 m in diameter and 5.7 m high cylindrical vessel were chosen as a benchmark problem to validate CFD codes and combustion models used for prediction of hazards in safety engineering. Simulations of two particular experiments with approximately the same amount of hydrogen were conducted: deflagration of a uniform 12.8% vol. hydrogen mixture and deflagration of a non-uniform hydrogen mixture, corresponding to an average 12.6 % vol. hydrogen concentration (27% at the top of the vessel, 2.5% at the bottom of the vessel) with ignition at the top of the vessel in both cases. The comparison of the simulation results for pressure and flame dynamics against the experimental data is reported.