The transient changes in the temperature of a solid material heated by radiation were obtained by numerical calculations using an unsteady one-dimensional (I-D) energy equation. The radiative loss of heat from the surface of a heated solid increases with an increase in the radiation flux. Therefore, the net blackbody radiation into a semi-infinite solid keeps a higher proportion of the initial flux at a lower dimensionless initial flux I-0(0)alpha k(T-b - T-0). In the case of bidirectional radiation to a finite solid, the relatively homogeneous heating can be accomplished at the optical thickness alpha L congruent to 10 degrees. The heating rate is very low at alpha L less than or similar to 10(-1) resulting from the absorption of a small part of the radiation energy. On the other hand, the heating rate is high at aL 101, but the heating is limited to a region near the wall, which results in an inhomogeneous temperature profile. At aL 102, the temperature profiles become identical with that obtained for aL -> infinity. (c) 2005 American Institute of Chemical Engineers AIChE J, 52: 478-483, 2006