Liquid liquid equilibrium data of 5-methyl-2-pyrazinecarboxylic acid (MPCA) and H(2)SO(4) in water + organic solvent systems with the reactive extractant trioctylamine (TOA) are presented together with the model of equilibrium. Dissociation constants of both acids were also determined. The addition of 1 kmol.m(-3) of Na(2)SO(4) decreased the pK(a) of MPCA and pK(a2) of H(2)SO(4) from 3.09 to 2.87 and from 2.10 to 0.908, respectively. In the extraction of MPCA, the overloading of TOA indicated the formation of (p, l) complexes (HMPCA)(p)TOA with p = 1 to 3. The (1, 1) complex is more stable in n-alkanes with isodecanol. Other (p, l) complexes are more stable in xylene. H(2)SO(4) with TOA in xylene form (r, q) complexes, (SO(4)(2-))(q-r)(HSO(4)(-))(2r-q)(HTOA(+))(q). At low H(2)SO(4) concentrations, a (1, 2) complex, TOA/sulfate, is formed. On increasing the acid concentration, the expected (1, 1) complex, TOA/hydrogen sulfate, aggregates, probably immediately, to complexes (2, 3) and (3, 3). When the ionic strength was increased by Na(2)SO(4), the extraction of H(2)SO(4) was less effective. High water content of the solvent loaded with H(2)SO(4) with the stoichiometry (1, 2) suggests the formation of reverse micelles similarly as observed in ionic liquids.