The coexistence of tricaprylmethylammonium nitrate ([A336][NO3]) and Di(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP) has been found an obvious synergistic extraction effect for rare earths nitrates. In this work, synergistic extraction behaviors without any diluent for rare earth nitrates based on tri(hexyl)tetradecylphosphonium chloride (Cyphos (R) IL 101, [P66614][Cl]) and DEHEHP have been further investigated. Pr(III) was still used as a model RE, and the maximum synergistic extraction distribution ratio was obtained at a volume ratio of 2:3 for [P66614][Cl] and DEHEHP. The synergistic enhancement coefficients have been tested for the whole rare earths. Results show that there is an obvious synergistic enhancement effect for light REs (La-Sm) and an anti-synergistic effect for Gd-Lu including Y. The effects of acidity, anions and metal ion concentrations on the extraction have been investigated. The extraction mechanism has been confirmed to be the typical ion-association reactions. The maximum loading capacity is 0.78 mol/L Pr at 293.15 K. The extraction ability of [P66614][Cl]-DEHEHP is superior to that of reported [A336][NO3]-DEHEHP, and is also greater than that of common acidic organophosphate-kerosene extraction system. Stripping, recyclability, and selectivity between REs and non-REs, has also been investigated. (C) 2017 Elsevier B.V. All rights reserved.