Extraction radiochemical investigations of ternary ion-association thiocyanate complexes of Co(II) with nitroblue tetrazolium chloride (NBT) [(3,3-dianizol-4,4-bis) 2-(4-nitrophenyl)-5-phenylnitrotetrazolium chloride] and triphenyltetrazolium chloride (TTC) [2,3,5-triphenyltetrazolium chloride] have been carried out. Molar ratios of the reacting components have been determined as NBT/[Co (SCN)4]2–=11 and TTC/[Co(SCN)4]2–=21. Using a chemical model and the method of Likussar — Boltz we have determined the values , Kex and KD, characterizing the extraction process. The results have been statistically treated. The relative standard deviation Sr has been calculated at a confidence level of 95%.