The synthesis, structural characterization, and magnetic property studies of five new transition metal (M = Co, Ni) phosphonate-based cages are reported. Three substituted phenyl and benzyl phosphonate ligands [RPO3H2; R, = p-tert-butylbenzyl, R-2 = p-tert-butylphenyl, R-3 = 3-chlorobenzyl] were synthesized and employed to seek out high-nuclearity cages. Complexes 1-3 are quasi-isostructural and feature a dodecanuclear metal-oxo core having the general molecular formula of [M-12(mu(3)-OH)(4) (O3PR)(4)((O2CBu)-Bu-t)(6) ((HO2CBu)-Bu-t)(6)(HCO3)(6)] {M = Co, Ni and R = R-1 for 1 (CO12), R-2 for 2, 3 (Co-12, Ni-12)}. The twelve metal centers are arranged at the vertices of a truncated tetrahedron in a manner similar to Keggin ion. Complex 4 is an octanuclear nickel phosphonate cage [Ni-8(mu(3)-OH)(4) (OMe)(2)(O3PR1)(2) ((O2CBu)-Bu-t)(6)((HO2CBu)-Bu-t)(8)], and complex 5 represents a pentadecanuclear cobalt phosphonate cage, [Co-15(chp)(8)(chpH) (O3PR3)(8)((O2CBu)-Bu-t)(6)], where chpH = 6-chloro-2-hydroxypyridine. Structural investigation reveals some interesting geometrical features in the molecular cores, which may provide new models in single molecular magnetic materials. Magnetic property measurements of compounds 1-5 indicate the coexistence of both antiferromagnetic and ferromagnetic interactions between magnetic centers for all cages.