An experimental study on laminar flame speed, Markstein length, and the onset of cellular instability was conducted by varying the equivalence ratio and ethylene/methane mixing ratio in spherically propagating premixed flames at ambient temperature and elevated pressures up to 0.8 MPa. Unstretched laminar burning velocities were first validated for methane-air flames by optimizing the range of the flame radius in testing linear and non-linear extrapolation models, and subsequently comparing the results with those simulated using four kinetic mechanisms. Based on the results, unstretched laminar burning velocities were determined for premixed flames of methane/ethylene mixture fuels. The predictability of theoretical Markstein lengths was appreciated by adopting a composite solution of the heat-release-weighted Lewis number and the temperature-dependent Zel'dovich number. Measured Markstein lengths were compared with those predicted based on a composite model for laminar flame speeds against flame radius. Depending on the fuels (methane or methane/ethylene mixture), pressure, and equivalence ratio, the predictability of the model varied. For methane-air flames, cellular instabilities were not observed within the observation window at pressures up to 0.6 MPa. Cell formation, caused by hydrodynamic instability, was enhanced by an increase in the ethylene ratio and chamber pressure. Theoretical critical flame radii for the onset of cellular instability predicted by the composite model were consistent with the measured ones for both lean and rich mixtures. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.