Thermodiffusive and hydrodynamic instabilities cause a departure from laminar combustion and, as such, are responsible for flame self-acceleration behavior. Due to the presence of hydrogen, syngas is a fuel prone to instabilities and is frequently diluted with CO2 or co-fired with CH4. This paper, thus presents an experimental study on how syngas mixed with CO2 and/or CH4 influence the onset of both kinds of instability. First, the results show that the laminar flame thickness is the controlling parameter in determining the critical radius at which cellularity appeared when CH4 is added to syngas. Moreover, higher levels of CO2 dilution translated into a constant critical radius, illustrating that the thermodiffusive mechanism is counterbalanced by the hydrodynamic one. To help differentiate between both kinds of instability, it is suggested herein to use the coefficient of self-acceleration or the ratio of the flame speed at the critical flame radius on the laminar flame speed. Finally, a correlation predicting the onset of cellularity is proposed based on the equation format proposed by Jomaas et al. (2007), derived from the stability analysis of a spherically expending flame. The correlation expresses the critical Peclet as a function of hydrodynamic and thermodiffusive instabilities and was successfully validated against experimental data from this study and the literature.