Synergistic solvent extraction of nickel by 2-hydroxy-5-nonylacetophenone oxime (Mextral 84H, denoted as HL) mixed with organic acids (denoted as HA) containing neodecanoic acid (Versatic 10) and bis(2-ethylhexyl) phosphoric acid (P204) diluted in aliphatics was comparatively studied. The synergistic effects on nickel with Mextral 84H/Versatic 10 and Mextral 84H/P204 systems were investigated by extraction pH isotherm and equimolar method. The nickel-organic extractants species structure and stoichiometry were determined by slope analysis, Fourier transform infrared (FT-IR) spectrometry and ultraviolet and visible (UV-Vis) spectrometry. The results showed that synergistic effects on nickel with a negligible shift of 0.06 pH units in the Mextral 84H/Versatic 10 system and a significant shift of 0.79 pH units in the Mextral 84H/P204 system were observed compared with single Mextral 84H system. Versatic 10 has little effect on the nickel extraction. The square-planar complex with a formation of NiL2 in the Mextral 84H/Versatic 10 system was proposed, the same as that in the single Mextral 84H system. P204 participates in the reaction between nickel and Mextral 84H, probably forming an octahedral complex with a formation of Ni(H(2)A(2)L(2)). The synergistic effect could result from this formation of more stable and hydrophobic nickel adducts, which promote the distribution ratio of nickel.