A novel unsymmetrical diglycolamide (DGA), namely N,N-dihexyl-N',N'-didecyldiglycolamide (DHD(2)DGA), was synthesized for the extraction of Nd(III), Sm(III), Gd(III), Dy(III), Ho(III) and Er(III) in nitric acid solutions. The effects of HNO3, HCl or H2SO4 on the distribution ratio of the investigated metal ions were investigated. A systematic investigation was carried out using DHD(2)DGA in n-dodecane from nitric acid solutions. The main extracted species of Nd(III), Sm(III), Gd(III), Dy(III), Ho(III) and Er(III) were studied at different nitric acid concentrations by the slope analysis method and the enthalpy change of the extraction reactions was determined. The IR spectra of the extracted species were also investigated. The values of thermodynamic functions for the extraction of the lanthanides were calculated. The obtained results indicated that the effective separation of Ln(III) from Fe(III), Ni(II), Co(II) and Cs(I) can be obtained.