Interaction of 12-B heteropolyanions, [XMo₁₂O₄₂]⁸⁻, whereX=Ce, Th and U, with tetravalent transition metal ions Mn^IV and V^IV results in the formation of three well defined complexes, viz., (NH₄)₄[Mn^IVThMo₁₂O₄₂]·10H₂O (I), (NH₄)₄[Mn^IVUMo₁₂O₄₂]·8H₂O (II) and (NH₄)₄[V^IVCeMo₁₂O₄₂]·8H₂O (III), which are characterised by IR, Raman and EPR spectroscopies. Negative shift in the stretching frequency of Mo-O_t bond in IR spectra and appearance of a strong band at 818 cm⁻¹ in Raman spectra support the formation of these complexes. EPR spectra of complexes (I) and (II) show a predominant signal atg=4 and a weak signal atg=2 (with hyperfine coupling from⁵⁵Mn), characteristic of d³ system, (S=3/2 andI=5/2) with large zero field splitting, showing distortion from octahedral symmetry. EPR parameters of the vanadium complex (III) show thed-electron to be more localised on vanadium(IV) supporting the formulation of vanadium not to be a part of the “Mo₁₂” unit. The oxidising property of complexes (I) and (II) is studied using glucose as the substrate.