Flame acceleration experiments were performed in a 10 cm inner-diameter tube filled with evenly spaced 0.43 blockage ratio orifice plates. The critical mixture composition required for flame acceleration to a fast-flame was measured for four methane/hydrogen fuel-air mixtures at initial temperatures of 298 K, 423 K, and 573 K. These conditions provide a large range in the Zeldovich number between 12 and 28, where the Zeldovich number was calculated from the laminar burning velocity obtained from 1-D flame simulations. The data collapsed very well when the expansion ratio across the flame (calculated at the critical condition) was plotted versus the Zeldovich number. This is consistent with correlation proposed by Dorofeev [7], that was based on experimental data obtained over a narrower Zeldovich number range. For pure hydrogen fuel, the critical expansion ratio was found to be between 2 and 4, and for pure methane the critical expansion ratio was as high as 8, for an initial temperature of 573 K.