An ignition model is developed for quasi-homogeneous propellants consisting of two perfectly mixed compnents which decompose independently in the condensed phase and then react in the gas phase. The model describes the heat propagation in the condensed and gas phases as well as the diffusion of components in the gas phase. The transition processes from ignition to self-sustaining combustion under pulsed irradiation are studied numerically for the simplest case of a monopropellant. It is shown that the critical value of heat flux for stable ignition of monopropellant and the shape of the stable ignition peninsula in the exposure time-heat flux coordinates depend on the extinction criterion.