Distribution of nicotinic 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 and the phase ratio of 1:1 (v/v). Cyclic alcohol/amine system yields the largest synergistic extraction efficiency. The strength of the complex solvation has been found to be reasonably high for halogenated aliphatic hydrocarbons and nitrobenzene promoting mainly the formation of acid(1)-amine(1) structure. The influence of the acid structure over distribution has been evaluated through comparing the extractabilities of six acids containing different functional groups, i.e., benzoic, valeric, formic, levulinic, acetic and nicotinic acids. The results were correlated using various versions of the mass action law, i.e., a modified Langmuir equilibrium model and a chemodel approach comprising the formation of one or two acid-amine aggregated structures. (C) 2001 Elsevier Science B.V. All rights reserved.