Spectral properties of GRB's observed by PHEBUS

More than 180 gamma-ray bursts have been observed by the PHEBUS instrument aboard GRANAT observatory since its launch in December, 1989. A large database of the burst spectra in 0.1-100 MeV energy range has been accumulated that shows a wide variety of their evolution. Among the great majority of events displaying standard hard to soft evolution of spectra some bursts exhibit unusual behaviour with hardness ratio rising with time. We present here the brightest examples for both types of events and discuss also the question of the presence of lines in burst spectra.