The formation of the mixed complex UO2(TOPO)(TBP)(NO3)2 is reported by31P-NMR spectroscopy for the first time. The extraction of the uranyl nitrate by a TBP and TOPO mixture was simulated in a NMR vial and monitored in situ. The equilibrium constant values of the processes in which the mixed complex is involved, i.e.:KR, for the redistribution of TBP and TOPO ligands between their uranyl complexes,KM, for the formation from consititutive molecules,Ks, for the substitution of the uranyl coordinated TBP by TOPO are determined. The redistribution constant,Kr at 25°C (4.43±0.5) and the changes of the thermodynamic variables H (0.59±0.14 kcal/mol). S (5.04±0.51 cal/mol·degree) have values close to the statistical values, suggesting a random nature of this process. TheKS value (4.5·103) reveals the much greater affinity for uranium(VI) of TOPO relative to TBP.