We present a Nb K-edge EXAFS study of carbon-and alumina-supported niobium sulfide hydrotreatment catalysts. NbS2 and NbS3 entities have been evidenced on the carbon support, and NbS2 and Nb1-yS ones on the alumina Support. EXAFS spectra of carbon-supported species, which are highly disordered, were analyzed through an asymmetric distance distribution model. Considering their poor dispersion, Nb1-yS species can be thought to be intrinsically highly active. Their activity, and that of NbS3 entities as well, can possibly be related to cation clustering. Niobium sulfides appear as very sensitive to support effects.