A series of novel manganese(II)-substituted polyoxometalates, [(Mn-II(H2O)(3))(2)(WO2)(2)(BiW9O33)(2)](10-) (1), [(Mn-II(H2O))(3)(SbW9O33)(2)](12-) (2), and [(Mn-II(H2O)(3))(2)(Mn-II(H2O)(2))(2)(TeW9O33)(2)](8-) (3), were synthesized and characterized by X-ray structure analyses. The use of these oxidatively and solvolytically stable heteropolyanions as homogeneous catalysts for the epoxidation of dienes was investigated by gas chromatography/mass spectrometry, IR spectroscopy, UV-visible studies, and cyclic voltammetric measurements. The catalytic performance is exemplified by the model substrate (R)-(+)-limonene, at ambient temperatures in a biphasic system, with excellent regioselectivities, >99%, and very high turnovers even with only a small molar excess of hydrogen peroxide.