An experimental study was performed to determine the requirements for sustained propagation of spark-ignited hydrogen-air and butane-air flames at atmospheric and elevated pressures. Results show that sustained propagation is always possible for mixtures whose Lewis number is less than unity, as long as a flame can be initially established. However, for mixtures whose Lewis number is greater than unity, sustained propagation depends on whether the initially ignited flame can attain a minimum radius. This minimum radius was determined for mixtures of different equivalence ratios and pressures, and was found to agree moderately well with the theoretically predicted critical radius beyond which there is no quasi-steady Propagation of the spherical flame. The essential roles of pressure, solution for the adiabatic, detailed chemistry, and the need to use local values in the quantitative evaluation of the flame response parameters are emphasized. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.