Interaction of 12-B heteropolyanions, [XMo12O42]8−, whereX=Ce, Th and U, with tetravalent transition metal ions MnIV and VIV results in the formation of three well defined complexes, viz., (NH4)4[MnIVThMo12O42]·10H2O (I), (NH4)4[MnIVUMo12O42]·8H2O (II) and (NH4)4[VIVCeMo12O42]·8H2O (III), which are characterised by IR, Raman and EPR spectroscopies. Negative shift in the stretching frequency of Mo-Ot 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.