The dissociation equilibrium constant (pK(a)) of an extractant is one of the main factors determining the reactive extraction behavior. A quantitative structure-property relationship (QSPR) was investigated for predicting the pKa of some pure extractants and the apparent basicity (pK(a,B)) of three typical mixture solvents, trioctylamine (TOA)/hexane, TOA/1-octanol, and TOA/methyl isobutyl ketone (MIBK). Novel QSPR models for the pK(a) of a pure extractant and the pK(a,B) of a mixture solvent were set up. The novel QSPR models include the concentration of extractant in the solvent and three kinds of molecular connectivity indices of extractant and diluent. The calculated values from the models of the pure extractant and mixture solvents show good consistency with experimental values.