Thenoyl trifluoroacetone (HTTA) has been used by workers in England and the U. S. as the acid in combination with neutral organophosphates (S) to demonstrate a strong (synergistic) enhancement of extraction of metallic species. This enhancement using HTTA is exhibited by many metal species and is much larger (up to 108) than that obtained with the dialkyl phosphoric acids (up to 102). This effect is increased as S is changed from the neutral alkyl phosphates, through phosphonates, to the phosphine oxides. Examples of synergism with HTTA and S are also given where S, the neutral additive, contains no phosphorus and is an amide, alcohol, or ketone. Tracer work has established the composition of over 30 extracting species containing di, tri, and tetravalent metals of the general formula M(TTA)xSy, where x is the valency of M, and y varies between 1 and 3. Confirmation of these formulas has, in a number of instances, been obtained by ultraviolet spectrophotometry on tenth molar solutions and by analysis of anhydrous microcrystalline solids isolated from solution. Stability constants have also been ascertained for many of these synergistic species. Infrared work indicates the likelihood of some of the chelate Ugands becoming monodentate in the synergistic species with direct bonding of S to the metal. If an excess of neutral ester S is added to a synergistic system, antisynergism, that is the reverse of synergism, occurs and decreases of 108 in extraction coefficient can occur. The degree of antisynergism depends on the quantity and nature of S. S could be an alcohol, amide, ketone, ether, ester (including organophosphorus esters) or, in fact, a so called “inert” diluent such as benzene or chloroform. The cause of these very large antisynergistic effects is bound up with the water content of the neutral additive S and of the diluent. There is ultraviolet spectral evidence that the anhydrous species is destroyed with formation of a hydrated TTA species.