Experiments were conducted in a constant pressure combustion chamber using highspeed schlieren imaging to investigate the effects of hydrocarbon addition on cellular instabilities of syngas-air flames at room temperature and elevated pressures. The measured laminar burning velocities were compared with the predicted results computed using some reliable kinetic mechanisms with detailed transport and chemistry. The cellular instabilities for the hydrocarbon-added syngas-air flames were interpreted and evaluated in the viewpoint of the hydrodynamic and diffusional-thermal instabilities. Also, experimentally measured Peclet numbers were compared with the predicted results for fuel-lean flames. Experimental results showed that the laminar burning velocities decreased significantly with the increase amount of hydrocarbon addition in the reactant mixtures. With propane and butane additions, the propensity for cells formation was significantly diminished in both hydrodynamic and diffusional-thermal instabilities, whereas the cellular instabilities for methane-added syngas-air flames are not suppressed. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.