The extraction mechanism of palladium (II) with the acidic chelating extractant 1-phenyl-3-methyl-4-propionylpyrazolone-5-one (HPMPP) and the tertiary amine (R 1 R 2 R 3 N) has been studied by spectrophotometry. A noticeable antagonistic extraction effect was observed in the extraction system under the given conditions. To understand this phenomenon, a preliminary work was performed to explain the mechanism of this reaction. According to the theory of the corresponding solutions (TCS), the association reaction between HPHBP and R 1 R 2 R 3 N was represented and discussed in the organic phase. An associated species HPMPP Á NR 1 R 2 R 3 formed through hydrogen bonding in chloro-SOLVENT EXTRACTION AND ION EXCHANGE, 19(5), 925-938 (2001) 925 ORDER REPRINTS form medium might be the main reason why an antagonistic extraction effect occurs. The association constant between HPMPP and R 1 R 2 R 3 N was calculated to be 2.2 AE 0.