Di-2-methylnonylphosphoric acid (HA) as an extractant was investigated for the extraction of divalent metal ions into heptane at an aqueous ionic strength of 0.10 mol dm(-3) (NaClO4) and at 25 degreesC. The extraction ability of metal complexes decreased in the order Cd(II) approximate to Mn(II) > Cu(II) > Co(II) > Ni(II). The metal complexes extracted were found to be all monomeric species for these metal ions using a slope analysis method. The curve fitting method was also applied to analyze the types of metal complexes extracted: MnA(2)3HA and MnA(2)4HA for manganese(II), CdA(2)3HA and CdA(2)4HA for cadmium(II), CuA(2)2HA and CuA(2)3HA for copper(II), CoA(2)3HA and CoA(2)4HA for cobalt(II) and NiA(2)4HA and NiA(2)5HA for nickel(II). Further, the extraction constants for the extracted metal complexes were evaluated.