If ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X–ray spectrum. Such features can be detected by existing and planned X–ray satellites, as long as the X–ray afterglow is observed after a few minutes from the burst. Detection of these X–ray features will make possible the determination of the redshift of ray bursts even when their optical afterglows are severely dimmed by extinction. For further detail see Ghisellini et al. (1999).