Quasi-thermal Comptonization in internal shocks formed between relativistic shells can account for the high-energy emission of gamma-ray bursts. This is in fact the dominant cooling mechanism if the typical energy bf the emitting particles is achieved either through the balance between heating and cooling or as a result of electron-positron pair production. Both processes yield subrelativistic or mildly relativistic energies. In this case, the synchrotron spectrum is self-absorbed, providing the soft-seed photons for the Comptonization process, whose spectrum is flat [F(nu) similar to const], ending in either an exponential cutoff or a Wien peak, depending on the scattering optical depth of the emitting particles. The self-consistent particle energy and optical depth are estimated and found in agreement with the observed spectra.
