A study with PIV-measurements for gas explosion in hydrogen-air mixtures is presented in this paper. The present work is part of an ongoing research project. The experiments are performed with hydrogen-air mixture at atmospheric pressure and room temperature. The experimental rig is a square channel with 4.5 X 2.0 cm2 cross section, 30 cm long with a single cylindrical obstacle of blockage ratio 1/3. The equipment used for the PIV-measurements was a Firefly diode laser from Oxford lasers, Photron SA-Z high speed camera and a particle seeder producing 1 µm droplets of water. The gas concentrations used in the experiments was between 14 and 17% hydrogen in air. The resulting explosion can be characterized as slow. Explosions in the gas mixtures at the highest hydrogen concentration produced measured flow velocities of up to 17 m/s as the flame passed the obstacle. Similar velocities was also measured one channel height behind the obstacle. The flow vortices produced behind the obstacle seemed to give separation between the liquid droplets and gas flow. The experimental results can be used for reference in validation of CFD-codes.