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Magnetization and magnetic susceptibility of dimeric polyoxoanions [M-4(H2O)(2)(GeW9O34)(2)](12-) (M = Mn2+, Cu2+) were investigated using an isotropic exchange coupling model with two coupling constant (J and Y) within the spin quartet formed by M4O16 (M = Mn2+, Cu2+). The exchange couplings were found to be antiferromagnetic for both samples with the values (J/k(B) = -0.91 ± 0.04 K and J'k(B) = - 1.18 ± 0.02 K) and (J/k(B) = - 13.5 ± 0.5 K and J'/ k(B) = -66 ± 4 K) for the samples containing Mn2+ and Cu2+, respectively. Furthermore, X- and Q-Band powder electron paramagnetic resonance (EPR) measurements on the sample containing Cu2+ were investigated and the experimental spectra were simulated using a triplet spin state S = 1 and I = 3, and including a small axial distortion D term. This leads to spin-Hamiltonian parameters of: | D| = 0.44 ± 0.01 GHz, g|| 2.415 ± 0.005, g⊥ = 2.070 ± 0.005, A(ii) = (40 ± 1) G. © 2004 Elsevier B.V. All rights reserved.