Cationic replacements in the arsenates NaCa 2 M 2 > > 2 (AsO 4 ) 3 (M 2 > > ؍ Mg, Ni, Co) have a strong influence on their garnetalluaudite polymorphism. The preferential formation of an alluaudite phase (at the expense of the garnet phase) is induced by the following cationic modifications: (i) The replacement of Na> > by larger cations such as Ag> >, K> >, or Tl> >. The cation distribution is partly disordered in the Na and Ag compounds and ordered in the K and Tl compounds. (ii) The formation of vacancies through the (Na> > or Li> >)-Ca 2 > > replacement, namely (Na or Li) 1 \ \ x ) x/2 Ca 2 > > x/2 Mg 2 (AsO 4 ) 3 . The formation of a pure defect garnet phase is limited to small x values (x ؍ 0.1 or 0.2) whereas a full series of defect alluaudite phases (terminating at Ca 2.5 Mg 2 (AsO 4 ) 3 , x ؍ 1) can be obtained. (iii) The replacement of Ca 2 > > by Mg 2 > > or Co 2 > >, leading to the new alluaudite-like compounds NaCaMg 3 (AsO 4 ) 3 , NaCaCo 3 (AsO 4 ) 3 , and NaCo 4 (AsO 4 ) 3 . Most of these results can be explained by the existence, in the alluaudite structure, of three crystallographic sites X(2), X(1), and M(1), which are respectively larger, approximately equal to, or smaller than the dodecahedral sites of the garnet structure. A garnet-alluaudite transformation has not been observed in the corresponding vanadates, in which the garnet structure appears very stable.