The influence of di-n-butyl phosphoric acid (HDBP) upon extraction of uranium(VI) by tri-n-butyl phosphate (TBP) from 0·5–3·0 M nitric acid solutions has been studied. It has been shown that the uranium(VI) distribution coefficientDufor extraction by 1·1 M TBP in tri-decane or xylene is increased when HDBP is present in the organic phase. For iso-molar solutions of (TBP + HDBP) with a total concentration of 0·36 M, and Uranium(VI) aqueous concentration up to 10–20  g/1, a maximum value of Du is observed when TBP/HDBP =1; for higher U(VI) concentration the maximum gradually disappears, with Du growing monotonically with the HDBP content in the organic phase. Uranium(VI) absorption spectra for 1·1 M TBP in tri-decane or xylene, containing HDBP, provide evidence for the formation of compounds, of which composition is intermediate between uranyl nitrate - TBP disolvate and the U(VI) - HDBP complex. It is proposed that these intermediate compounds are UO2(NO3)2, HDBP.TBP and UO2(NO3)2(HDBP)2.