The compluxation of Pu(IV) by sulfate has been studied (in 1M acid, 2Af ionic strength Perchlorate media) by a TTA (thenoyltrifluoroacetone) extraction procedure. Experiments were performed at 0.0, 10.0, 25.0, and 35.0°C to allow calculation of the enthalpy and entropy of the complexation reactions. From the thermodynamic Parameters and comparison with stability constants determined in a previous study at 2^ acidity and ionic strength, the postulated reactions are