The synergistic extraction of copper (II) from 1M NaN03 aqueous solutions with acidic chelating extractants HL = 3-phenyl-4-acylisoxazol-5-ones (acyl = benzoyl, "HPBI", and p-tert-butylbenzoyl, "HPtbBI") and 1-phenyl-3-methyl-4-acylpyrazol-5-ones (acyl = benzoyl, “HPMBP”, p-tert-butylbenzoyl, “HPMtbBP” and stearoyl, "HPMSP"), in the absence and presence of trx-n-octylphosphine oxide (TOPO) in toluene at 25°0, has been studied. The TOPO-HL interaction, which is one order of magnitude stronger with 4-acylisoxazol-5-ones than with 4-acylpyrazol-5-ones, strongly influences the synergistic extraction efficiency. The extracted species are CuL2and CuL2(T0P0). Because of their higher acidity, the extractions performed with 4-acylisoxazol-5-ones occur at a lower pH range than with 4-acylpyrazol-5-ones.