W hen a laminar flame front becomes cellular, the flame front area increases and the rate at which the unburnt gases are consumed is increased. If this is not taken into account when modelling vented or non-vented explosions, the pressures that vessels containing explosions are subject to can be greatly under-predicted. For most explosion models, hydrodynamic instabilities are considered as the only driving force for the formation of cellular flames. This paper shows that the transition to cellular flame propagation can occur earlier due to then-no-diffusive instabilities. To predict when thermo-diffusive instabilities should be considered as the main driving force for cellular flame transition, the Lewis number of the reactant mixture should be compared to a critical value. Calculations of the Lewis number for a range of different fuel/air mixtures that are in common use are presented. The fuels studied are methane, ethane, propane and n-butane. For each of these fuel/air mixtures the critical Lewis number was evaluated using an expression from asymptotic analyses. It is shown that the critical Lewis number based on this type of analysis is under-predicted. This is because the flame thickness is not included. It is proposed that a modified form of the critical Lewis number be used which is greater by an order of 1/Zeldovich number.