The preheat of the cold substrate by the x rays generated in the coronal region of a laser produced aluminum plasma was calculated for a trapezoidal laser pulse in the intensity rangeI=1013–8×1014W/cm2. The computer code incorporates nonlocal thermodynamic equilibrium (non‐LTE) radiation emission rates in the hot region and photoionization as the main photoabsorbing process in the cold compressed region. The temperature and the density variations of the photoionization cross section were accounted for. Black‐body radiation diffusion was also included. The results indicate the formation of a well‐defined radiation heated zone in the back portion (close to the ablation surface) of the shock wave. Comparisons were made on the effects of the x‐ray radiation and the black‐body diffusion on the shock wave propagation. The similarities and differences between x‐ray and fast electron preheat are also discussed. In a different set of computations an attempt was made to simulate the measurements of McLeanetal. [Phys. Rev. Lett.45, 1246 (1980)] of the backsurface temperature of laser irradiated aluminum foils. The computational results agree favorably with those of the experiments.