Experimental study on the laminar burning velocities and the onset of cellular instabilities of propane-hydrogen-air mixtures with spherically expanding flames was conducted at elevated pressures and temperatures and different hydrogen fractions at the equivalence ratio of 0.8 and 1.2. The results show that the unstretched flame propagation speed and the unstretched laminar burning velocity increase with the increase of hydrogen fraction and initial temperature, and they decrease with the increase of initial pressure. An earlier onset of cellular instability and the decrease in the critical radius and the Markstein length are presented with the increase of initial pressure, indicating that the hydrodynamic instability is enhanced with the increase of initial pressure. At the equivalence ratio of 0.8, where the propane-air mixture is thermal-diffusion ally stable and the hydrogen-air mixture is thermal-diffusionally unstable, the critical radius and the Markstein length decrease significantly with the increase of hydrogen fraction, indicating that hydrogen addition will increase the diffusional-thermal and the hydrodynamic instability. At equivalence ratio of 1.2, where the propane-air mixture and hydrogen-air mixture are both thermal-diffusionally neutral, a moderate decrease in the critical radius and the Markstein length is presented. This indicates the increase of hydrodynamic instability as hydrogen is added. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.