An experimental study was carried out to determine the requirements for the successful ignition and sustained flame propagation of spark-ignited mixtures of hydrogen/air at atmospheric and elevated pressures. Results show that successful ignition depends on the ignition energy deposited, the Lewis number (Le) of the mixture, and the system pressure. In particular, sustained flame propagation is always possible for lean mixtures whose Le is less than unity, as long as a flame can be initially established. However, for rich mixtures whose Le is greater than unity, sustained propagation depends on whether the initially ignited flame can attain a minimum radius. This critical radius was determined for mixtures of different equivalence ratios and pressures, and was found to agree moderately well with the theoretically predicted minimum value beyond which there is no solution for adiabatic quasi-steady propagation.