Abstract: Five different acylpyrazolones, with various substituing groups, were used to study the extractibility of Ce(Ⅲ) at pH 3.1 into chloroform. The extractive ability decreases in the following order: (HTTAwas also included for comparison).1-Phenyl, 3-methyl, 4-trifluoroacetyl pyrazolone(5) (HAPZ-8113F)>1-phenyl-3-phenyl, 4-benzoyl pyrazolone(5) (HAPZ-888)>1-phenyl, 3-methyl, 4-benzoyl pyrazolone(5) (HAPZ-818)>1-phenyl, 3-methyl, 4-acetyl pyrazolone(5) (HAPZ-811)>2-thenoyl trifluoroacetone(HTTA)>l-phenyl, 3-methyl, 4-formyl pyrazolone(5) (HAPZ-810)The reasons for introducing the above symbols in the above parentheses to represent the various extractants are explained.From the slope method, although HTTAin HCl gave the extracted species as CeA₃, various acyl pyrazolones gave CeA₂·Cl·HAin the same experimental conditions. In HNO₃ system, HAPZ-8113Fgave CeA₃ however.The logarithms of the extraction equilibrium constants calculated from the experimental data are: HAPZ-8113F/Cl-0.08, HAPZ-818/Cl^--0.82, HAPZ-811/Cl^- -2.57, HTTA/Cl^- -9.05, HAPZ-8113F/NO₃^--4.05. The extraction with HAPZ-810 is very slight.The synergistic effect, when dioctylsulfoxide DOSOwas used in conjunction with various acylpyrazolones, was also studied. The effectiveness of synergism, when expressed as the ratio, D_max(HA + S): D_HA, was in the order,HAPZ-8113F>HTTA>HAPZ-888>HAPZ-818.Radioactive ¹⁴⁴Ce as fracer added to natural cerium, was used throughout the experiment.