Liquid liquid equilibrium data for butyric acid (BA) extraction by phosphonium decanoate ionic liquid (IL) were measured and modeled. An extended more general extraction mechanism taking into account differences in IL anion basicities was suggested. The overall extraction mechanism involving three submechanisms of competitive and noncompetitive extraction of BA and water coextraction was successfully applied to the system with phosphonium decanoate. Formation of (p, 1) complexes with p of up to 16 molecules of BA with 1 ion pair of IL was supposed. A 3 times lower value of the stability constant of the first IL BA bond in decanoate compared with phosphinate ILs could result in easier regeneration of acid-loaded IL in vacuum distillation. Aggregation and micellar structures in water-saturated IL were confirmed by dynamic light scattering measurements. The concentration dependence of equilibrium solvent-phase viscosities goes through a maximum that corresponds with a minimum in the dependencies of density and water content due to changes in solvent-phase composition. The tested IL can be an effective extractant for the separation of organic acids from production media of the fermentation process.