The present work reports that gas-phase radiation absorption is one of the critical mechanisms responsible for the gas-phase ignition of solid fuels which are exposed to a strong external thermal radiation. A one-dimensional transient model includes the effects of the gas-phase absorption of incident thermal radiation; in-depth absorption by the solid phase; and natural convection along the fuel surfaces. A numerical simulation confirms that the decomposition products significantly attenuate the incident radiation, which has been already observed experimentally. In spite of natural convection flows which dilute the absorbing gases and increase heat losses, absorbed radiant energy plays an important role in the induction of the gas-phase thermal runaway. Thus, ignitable domains are considerably different from those obtained without considerations of the gas-phase radiation absorption.