The extraction of Th(IV) from an aqueous solution of constant ionic strength with Versatic Acid-10 dissolved in kerosene was studied as a function of pH and the concentration of Th(IV) and organic acid. The effects of sulphate and chloride ions on the extraction of the Th(IV) were briefly examined. It was revealed that (ThR₄·2RH) is the predominant species of extracted Th(IV) in the Versatic Acid-10. The chemical equation and the apparent equilibrium constants thereof have been determined as follows: Th^(4+)+3(R₂H₂)_(org) = (ThR₄·2RH)_(org)+4H^+ K_(Th)=(ThR₄·2RH)_(org)·(H^+)⁴_(aq)/(Th^(4+)_(aq)·(R₂H₂)³_(org)=1.1×10^(-11)