The rational synthesis and the structural and magnetic characterization of three different nickel clusters encapsulated in Keggin trivacant fragments are presented. The three complexes show how it is possible to increase the nuclearity of the clusters (from 3 and 4 to 9) by slightly changing the synthetic conditions. These three anionic clusters crystallize as mixed salts of K+ and Na+. The trimeric complex [Ni-3(H2O)(3)PW10O39H2O](7-) (Ni-3) crystallizes in the triclinic space group P (1) over bar (a = 10.896(6) Angstrom, b = 12.869(5) Angstrom, c = 20.373(6) Angstrom, alpha = 94.67(6)degrees, beta = 101.12(8)degrees, gamma = 110.72(8)degrees, Z = 2) and presents a ferromagnetic triangular cluster. The tetranuclear complex [Ni-4(H2O)(2)(PW9O34)(2)](10-) (Ni-4) crystallizes in the monoclinic space group P2(1)/n (a = 11.824(5) Angstrom, b = 16.551(6) Angstrom, c = 21.074(5) Angstrom, beta = 100.38(2)degrees Z = 2) and presents a ferromagnetic rhomb-like cluster. The nonanuclear complex [Ni-9(OH)(3)(H2O)(6)(HPO4)(2)(PW9O34)(3)](16-) (Ni-9) crystallizes in the monoclinic space group P2(1)/c (a = 30.589(10) Angstrom, b = 12.521(6) Angstrom, c = 39.456(11) Angstrom, beta = 102.49(3)degrees, Z = 4) and presents an antiferromagnetic triangle formed by three ferromagnetic triangular clusters. The magnetic properties of the three complexes are discussed on the basis of their topologies and fitted according to an isotropic exchange model (for the three clusters) and including the local anisotropy of each center (for the Ni-3 and Ni-4 clusters).