The complexes formed between vanadium(III) and 6-methylpicolinic acid (H6Mepic) with small blood serum bioligands: lactic, oxalic, citric and phosphoric acids (HLac), (H2Ox), (H3Cit) and (H3PO4) were studied in aqueous solution by means of electromotive forcesmeasurements emf(H) at 25°C and 3.0mol dm−3 KCl as ionicmedium. The potentiometric data were analysed using the least-squares computational program LETAGROP, obtaining the complexes [V(6Mepic)(Lac)]+, V(OH)(6Mepic)(Lac), [V(OH)2(6Mepic)(Lac)]−, [V(OH)3(6Mepic)(Lac)]2− in the vanadium(III)–6Mepic-Lac system. In the case of the vanadium(III)–6Mepic-Ox system the complexes V(6Mepic)(Ox) and [V(6Mepic)2(Ox)]− were observed , in the vanadium(III)–6Mepic–H3PO4 system the complexes [V(6Mepic)(H2PO4)]+ and V(6Mepic)(HPO4) were detected, and finally in the vanadium(III)–6Mepic-Cit system the complexes V(6Mepic)(HCit), [V(6Mepic)(Cit)]− and [V(OH)(6Mepic)(Cit)]2− were observed . The respective stability constants of the complexes formed in aqueous solution were determined.