Cellular instability in spherical propagating hydrogen-methane-air flames was studied experimentally in a constant volume chamber at an equivalence ratio of 0.8 and mixture temperature of 350 K. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0 for mixture pressures up to 0.50 MPa. Cellular instability started earlier with an increase in the hydrogen mole fraction and mixture pressure. Self-acceleration of some of the propagating cellular flames was recorded and the acceleration increased with hydrogen mole fraction and mixture pressure. The unstretched laminar burning velocity was obtained from experiments and 1-D simulations of the outwardly propagating flames. Asymptotic theories gave a satisfactory qualitative prediction of the trends in the Markstein length, and the critical flame size for the onset of cellular instability. It was concluded that the Markstein length changed to a negative value at elevated pressure due to increased sensitivity of the burning velocity to thermo-diffusive effects. Copyright (c) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.