Three tridentate extractants and three masking reagents including O, N, and S donors have been developed and their properties are compared and discussed. The extractants are N,N,N′,N′-tetraoctyl-diglycolamide (TODGA), methylimino-N,N′-dioctylacetamide (MIDOA) and N,N,N′,N′-tetraoctyl-thiodiglycolamide (TDGA(C8)) together with masking agents having the same central frame but with short alkyl chains. The results of the present study indicate that TODGA can extract mainly hard acid metals belonging to groups 2-4,13-15 in the periodic table. MIDOA can extract soft acid metals and oxyanions (groups 5-10, 16), and TDGA can extract soft acid metals (groups 10-11). Some spectrophotometric studies (UV-vis., IR, and NMR) indicate the stoichiometry and the effect of donor atoms for metal-complexation. The ΔHf values, the heat generation during complex formation, obtained by chemical calculation by DFT theory show a reverse-correlation with their extraction ability. 1. Introduction Recently, tridentate extractants and ligands have been developed and studied worldwide [1-6]. Tridentate ligands can complex effectively with metals with 6 and 9 coordination sites with 1 : 2 and 1 : 3 metal-ligand complex stochiometries, rather than monodentate and bidentate extractants. The strong extractabilities and high complexation abilities for metals by tridentate ligands have been reported in many papers[1,4,6]. One of the most important tridentate extractant is TODGA (N,N,N′,N′-tetraoctyl-diglycolamide) [5]. This compound has three oxygen donors (two carbonyl oxygen donors and an ether oxygen atom) attached with diamide groups outside of the central frame. Based on the HSAB principle [7], oxygen donors are preferred for hard acid metals, metals belonging groups 2-4 in the periodic table. TODGA can dissolve in n-dodecane and shows a very high distribution ratio (D) for actinides (An) and lanthanides (Ln) from nitric acid. Therefore, TODGA is a useful extractant for metal separation from high level radioactive liquid waste [8-12]. TODGA is composed of C, H, O, N atoms and complete gasification takes place on combustion, thus producing less secondary waste. N and S donor extractants, which are similar to TODGA, can be produced by substitution of the O atom at the ether position in TODGA. The N-compound with a methyl group attached is termed as MIDOA (methylimino-N,N′-dioctylacetamide) and the S-compound is TDGA (N,N,N′,N′-tetraoctyl-thiodiglycolamide) [6, 13-16]. These extractants can be synthesized by changing the initial materials to methylimino-diacetic acid or thiodiacetic acid from the diglycol acid of TODGA. Both N and S donors are soft donors and complexing well with soft acid metals