The extraction of UO
, Am3+, and Th4+ by 1-phenyl-3-methyl-4-benzylpyrazolone with crown ethers was studies using 0.1M (NaClO4) aqueous phase and toluene. The crown ethers were 12C4, 15C5, 18C6, DB18C6 and DCH18C6. The synergic equilibrium constant did not show correlation between the cationic radii and the ether cavity size nor did the values follow a simple order of ether basicity. The ether basicity, steric effects, and the number of ether oxygens bound to the cation are the combined factors which seemingly determine the pattern of M(PMBP)n—CE interaction.