Decavanadate salts with nicotinamide (3-pyridinecarboxamide, 3-pca) and isonicotinamide (4-pyridinecarboxamide, 4-pca) in both neutral and protonated forms, (3-Hpca) 4 [H 2 V 10 O 28 ]·2H 2 O·2(3-pca) (complex I) and (4-Hpca) 4 [H 2 V 10 O 28 ]·2(4-pca) (complex II), have been synthesized and characterized by vibrational spectroscopy (infrared and Raman), thermogravimetric analysis (TGA), 51 V NMR, and single-crystal X-ray diffraction analysis. The effects of sodium decavanadate (henceforth called NaV 10 ) and compounds I and II on Escherichia coli, Giardia intestinalis, and Vero (African green monkey epithelial kidney) cells were evaluated. Enhanced growth inhibitory activity against E. coli cultures was observed upon treatment with products I and II when compared to that with NaV 10 (GI 50 values of 2.8, 4.0, and 11 mmol L -1 , respectively), as well as lower cell viability as measured by the intake of propidium iodide (PI). Exposure of Giardia trophozoites to NaV 10 and II revealed reduction in trophozoite viability (GI 50 values of ca. 10 μmol L -1 ) and affected the parasite adherence to both polystyrene culture tubes and a monolayer of Vero cells, even at low concentrations. A lesser effect on Giardia was shown for I. Furthermore, all three compounds were significantly less toxic to Vero cells than the reference drug, albendazole, employed in the treatment of giardiasis. Toxicity reports of oxidovanadium compounds toward Giardia are unprecedented and open a path to the development of new therapeutic agents.