  We present the spectral analysis of 14 gamma-ray bursts (GRB) X-ray
afterglows in order to investigate the properties of interstellar matter (ISM)
along the line of sight of GRB. We carried out a simultaneous analysis of the
NIR-optical and X-band for those afterglows with an optical counterpart too, in
order to evaluate and strongly constrain the absorption effect on the spectral
energy distribution due to dust extinction from GRB environment. We evaluated
the equivalent hydrogen column density Nh from X-ray spectroscopy and rest
frame visual extinction Av by assuming different type of ISM composition and
dust grain size distribution. From our analysis we obtained a distribution of
the GRB rest frame consistent with the one expected if GRB were embedded in a
Galactic-like molecular cloud. Moreover, values of the visual extinction
estimated from the simultaneous analysis of NIR-to-X band favour an environment
where small dust grain are destroyed by the interaction with the X-ray and UV
photons from GRB.
