Using the correlation between the equivalent width of the diffuse interstellar band (DIB) at 8620 A and the interstellar reddening reported by Munari (2000) GAIA could directly trace the interstellar extinction throughout the Galaxy. We checked for the magnitude and distance limitations of this method in 42 Galactic directions by simulating the RVS data on the stellar sample provided by the model of stellar population synthesis of the Galaxy (Robin et al. 2003). The simulation indicates that the imprint of the 8620 A DIB will be detected in the RVS spectra of stars with magnitudes up to V~16 with sufficient accuracy to trace not only the distribution of the interstellar medium but also the radial component of its kinematic motion, i.e. the radial velocity of the mass center of the dust cloud in the line of sight.
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