Two new mononuclear peroxo complexes of tungsten of the formula (gu)(2)[WO(2)(O(2))(2)] (1) and (gu)[WO(O(2))(2)(quin-2-c)] (2a) (where gu(+)=guanidinium ion, CN(3)H(6)(+) and quin-2-c=quinoline-2-carboxylate ion) have been synthesized and characterized by elemental analysis, infrared, Raman, UV-visible and (1)H NMR spectroscopies. The crystal structure of (gu)[WO(O(2))(2)(quin-2-c)].H(2)O (2b) determined by X-ray diffraction indicates that the side-on peroxo groups and the bidentate quinaldate ligand bind the W(VI) centre leading to an hepta coordination mode. The guanidinium ion occurring as a counterion and the hydrogen-bound interactions stabilize the complexes. The in vitro insulin-mimetic effect of the complexes has been evaluated by the inhibitory effect on free fatty acid release in isolated fat adipocytes treated with epinephrine. Moreover the niobate analogues, synthesized and characterized previously, (gu)(3)[Nb(O(2))(4)] and (gu)(2)[Nb(O(2))(3)(quin-2-c)].H(2)O have been tested for the insulin-like activity.