The radiative ignition behavior of two polymeric fuels—polystyrene and an epoxy—in oxygen/nitrogen mixtures has been determined experimentally. A CO 2 laser was adapted for use as the radiation source to simplify interpretation of the results. Ignition delay (as determined by first light emission) was measured as a function of oxygen percentage, pressure, radiant flux, and fuel absorptivity. The observed ignition delays, and the variations with pressure, oxygen fraction, and radiation intensity, conform to the predictions of a theoretical model based on the need to heat the surface to a critical ignition temperature. The most important finding is that ignition of a solid fuel by radiation is much slower than ignition by an identical heat flux applied by a hot gas source. Two main reasons for the retardation can be deduced from the experimental results: (1) the radiative transparency of the fuel, even for seemingly opaque substances, results in slower surface heating, and (2) the cool gas environment in the usual radiation ignition test suppresses the incipient ignition.