Extensive work is being carried on the extraction of rare earth elements due to their special chemical, metallurgical, optical, magnetic and nuclear properties and their use in advanced technologies as well as in nuclear industry. Different chemical processes are being applied for the extraction of rare earth elements from their ores and their mutual separation on laboratory scale as well as on commercial basis. However, these processes are facing problems such as large number of stages due to low separation factor, low efficiency and waste management. Keeping in view of these problems, in the present research work, a synergic extraction system comprising of picrolonic acid as an acidic chelating agent and oxygen based neutral donors, for the extraction / separation of rare earth elements has been studied. Synergic extraction of Ce(III), Nd(III), Eu(III), Tb(III), Tm(III) and Lu(III) as representative of trivalent lanthanides, using picrlonic acid (1-p-nitrophenyl-3-methyl-4- nitro-5-pyrazolone, HPA, pKa = 2.52) as acidic chelating agent with crown ether such as 18- crown-6 (18C6), Benzo-15crown-5 (B15C5), 12crown4 (12C4) as neutral oxo-donors in chloroform from aqueous buffer solution of pH 1-2 having ionic strength 0.1 mol L -1 (K + /H + , Cl - ) has been studied. Radiotracer technique using their appropriate radio-isotopes prepared in the research reactor of PINSTECH such as Ce 141 , Nd 147 , Sm 151 , Eu 152/154 , Tb 160 , Tm 170 , Lu 177 , Hg 203 , Fe 59 etc., were used for the quantification of metal ions in the aqueous and organic phases. Quantitative extraction (>98%) of these metal ions was observed only using HPA and B15C5 synergic mixture at pH 2 within five minutes and the extraction was increased with the increase in ionic radii of lanthanide ions. Composition of the extracted species was determined by slope analysis method and found to be Ln(PA)3.nS, where Ln represent lanthanide ion, PA conjugate base of HPA molecule and S as neutral oxo-donor. The value of n is 1 and 2. Among the various cations and anions tested for their influence on iiithe extraction these lanthanides only Fluoride, oxalate, Cu(II) , Fe(II) and Zn(II) had some deleterious effect. The proposed synergic system presented clean separation of lanthanide ions from mono, and various divalent metal ions especially alkali and alkaline earth metal ions. The effect of other neutral donors such as trioctylphosphineoxide (TOPO), triphenylphosphineoxide (TPPO), tributylphosphate (TBP) and triphenylphosphate (TPP) was also studied on the extraction of Eu(III). Quantitative extraction of Eu(III) was observed with TOPO, TPPO and TBP from aqueous phase of pH2. Synergic adduct composition was found to be Eu(PA) 3 TBP, Eu(PA) 3 .2TOPO and Eu(PA) 3 .2TPPO by slope analysis method. On the basis of the estimated values of the synergic coefficient, and extraction constants (log K ex ), the oxo-donor effect was found in the order of TOPO>TPPO>TBP. The effect of various diluents such as 1-octanol (ONL), 1-hexanol (HNL), 1-butanol (nBNL), 2-butanol (2-BNL), n-butylether (BE), dichloroethylether (DCEE), acetylacetone (ACAC), diisobutylketone (DIBK), cyclohexanone (CHN), benzene, toluene on the extraction of Eu(III) from aqueous solution of pH 1-2 using HPA as extractant has been studied. The extraction of Eu(III) using benzene and toluene was found to be negligible, with 1 & 2-butanol it was low (< 50%), where as with the other diluents studied, the extraction was quantitative at pH 2. On the basis of log K ex , the solvents can be arranged with respect to their extractability in the order ACAC > DIBK > BE > DCEE > ONL > HNL > CHN. To find the trend of lanthanide extraction within the series, three solvents CHN, ONL and DCEE as representative of ketones, alcohols and ethers, respectively, were selected for the extraction of Ce(III), Tb(III) and Lu(III) using HPA as chelating agent from aqueous solutions of pH 1-2, quantitative extraction was observed at pH2 and their extraction order was found to be Ce(III)>Tb(III) >Lu(III).The composition of the extracted adduct was found to be M(PA) 3 in CHN, ONL and M(PA) 3 .HPA in DCEE by slope analysis method. ivThe synergic mixture comprising HPA and B15C5 in benzene and toluene separately were studied for the extraction of Eu(III) from aqueous solution of pH 1-2 and quantitative extraction was observed at pH 1 with both the solvents. On the basis of their estimated values of synergic coefficient and log K ex , benzene was found to be better solvent than toluene. The composition of the synergic adduct was found to be Eu(PA) 3 .2B15C5 and proposed to be a sandwich type complex having one crown ether molecule on either side of the metal chelate bound to the central metal only through three oxygen atoms.