The extraction of lactic acid with different long chain (from C8 to C12) tertiary amines was studied, both theoretically and experimentally. In view to determine the concentration of free acid and bounded with extractant acid in the aqueous phase, the experimental results were treated by mass balance equations. The minimal and maximal fractions of the free acid and bounded acid were calculated for different compositions of the organic phase. The obtained results clearly showed that in general the interaction product concentration in the aqueous phase could not be neglected and, in some cases, reached more than 50% from the total acid concentration. The disparate influences of inert and active diluents (dodecane and oleyl alcohol, respectively) were elucidated. It was shown that the overall distribution coefficient depends not only on the extraction product concentration in the aqueous phase but also on the composition of the organic phase.