An experimental study on the effects of hydrogen addition on the instabilities of spherically expanding propane-air flames was conducted in a constant volume combustion vessel over a wide range of mixture compositions and initial temperatures and pressures. The measured laminar burning velocities were compared with those calculated values by using one dimensional freely propagating flames and a recently developed detailed kinetic mechanism. Good agreement was obtained between the experiment and calculation. The schlieren images show that for lean mixture combustion, hydrogen addition will increase the hydrodynamic instability due to the decreased flame thickness and increase the diffusional-thermal instability due to the decreased Lewis number. While for rich mixture combustion, the flame front is initially destabilized and later tends to the stabilized with the increase of hydrogen fraction. This is due to the competing effects of the hydrodynamic instability and the diffusional-thermal instability. (c) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.