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The influence of chemical B-site ordering between Sc 3+ and Nb 5+ cations on the properties of the Pb(Sc 1/2 Nb 1/2)O 3 (PSN) relaxor has been investigated. Depending of the degree of ordering, PSN exhibits different behaviours. For a completely disordered material, a relaxor-ferroelectric phase transition is observed at 379 K by DSC. Ordering between Sc 3+ and Nb 5+ cations on the B-site of the perovskite structure leads to a non-homogeneous material constituted of two phases: ordered and disordered phases. The phase transition temperature of the ordered phase is confirmed to be lower (346 K) than that of the disordered phase. It appears that the phase transition of the disordered phase is shifted to lower temperature owing to the presence of the ordered phase. Moreover, an additional short-range structural ordering is shown to exist in disordered PSN. This additional ordering, for which antiparallel displacements of Pb atoms could be involved, is favoured by chemical ordering between Sc 3+ and Nb 5+ and is long range in ordered PSN.