The self-acceleration of spherically expanding flames were investigated using a constant volume combustion chamber for CO/H-2/O-2/N-2 mixtures over a wide range of initial pressure from 0.2 to 0.6 MPa, CO/H-2 ratio from 50/50 to 10/90 and equivalence ratio from 0.4 to 1.5. The adiabatic flame temperature was kept constant by adjusting O-2/N-2 ratio at different equivalence ratios. Schlieren images were recorded to investigate the flame front evolution of spherically expanding flames. Local acceleration exponents were extracted using a proper equation to study the process of flame self-acceleration. Results show that the flame cells develop on the smooth flame fronts and finally reach fractal-like structures due to the hydrodynamic and diffusional-thermal instabilities, resulting in flame self-accelerative propagation. The critical Peclet number corresponding to the onset of self acceleration, Pe(cr) increases nonlinearly with the Markstein length, Ma. The observation further reveals that the onset of self-acceleration is mainly controlled by the diffusional-thermal effect. There exists two distinct flame propagation regimes in the self acceleration, namely quick transition accelerative and quasi self-similar accelerative regimes. The quick transition regime is controlled by the destabilization effect of hydrodynamic perturbation and stabilization effect of flame stretch. While the quasi self-similar regime is primarily affected by the cascading process of flame front cells controlled by hydrodynamic instability. The self-similar acceleration exponent, alpha(s) varies with the initial pressure and Lewis number, Le. The values of alpha(s) are measured to be 1.1-1.25 (smaller than 1.5), indicating the flame dose not attain self-turbulization. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.