A family of virtually isostructural tetra-capped epsilon-Keggin-type polyoxomolybdate(V) cluster compounds, [(Mo12O28)-O-V(mu(2)-OH)(10)(mu(3)-OH)(2){M-II(H2O)(3)}(4)]center dot nH(2)O ({(M4Mo12V)-Mo-II}, M = Ni, Co), exhibits magnetic-field-dependent optical response in their electronic absorption spectra in the 0-33 T range. On the basis of Effective Hamiltonian Crystal Field calculations, we find that the observed field-induced decrease in reflectance of these compounds can be related to the formally spin-forbidden on-site d-d excitations. We tentatively position the observed effect among other known magneto-optical effects and predict that similar features may be found for other {(M4Mo12V)-Mo-II} analogues.