Distribution of valeric acid between water and Alamine 336, a mixture of tertiary aliphatic amines dissolved in various (proton-donating and -accepting, polar and nonpolar) diluents, as well as a comparison with the extraction equilibria of pure diluent alone, have been studied at 298 K. The influence of the acid structure over distribution has been interpreted through comparing the extractabilities of four acids containing different functional groups, i.e. valeric, formic, levulinic and nicotinic acids. The results were correlated using a modified linear solvation energy relation (METLER) and various versions of the mass action law, i.e. a chemodel approach and a modified Langmuir equilibrium model comprising the formation of one or two acids per multiple amines aggregated structures. The reliability of the proposed extraction models has been analyzed statistically on the basis of the overall loading factor of amine, Z(t), using a log-ratio objective function.