Abstract: In this paper the extraction mechanisms of uranyl nitrate and thorium nitrate by DMHMP are studied. The compositions of the extraction complexes are UO2(NO3)2. 2DMHMP and Th(NO3)4·3DMHMP. At a constant ionic strength μ = 2, the equilibrium constants for their formation are lg KU = 4.6, lg KTh = 4.3 respectively. The measured values of the distribution ratio of extraction agree quite well with those obtained by Leden's method.Successive stability constants of the nitrate complexes of thorium by extraction with DMHMP are calculated by Lenden's graphical method and via a least squares technique using a PDP11/70 computer. Both results agree well. The successive stability constants of the complexes formed are: β1 = 8.0,β2 = 5.2,β3= 0.8,β4= 1.3. By comparison of equilibrium constants complex formation of DMHMP and TBP with U and Th, it can be expected, that DMHMP is superior to TBP for the extraction separation of uranium and thorium.