Experimental data on L/L equilibria in extraction of propionic acid (PA) from aqueous solutions by hydrophobic phosphonium ionic liquid (IL) tetradecyl(trihexyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate [C14C6C6C6P][BTMPP] and dodecane are presented. Modeling of equilibrium data indicated that extraction occurs via the same mechanism as in butyric acid extraction by this IL as well as by [C14C6C6C6P][decanoate]. Competitive extraction of PA and water and coextraction of PA with water are two mechanisms running in parallel, while the first one is more pronounced at lower PA concentrations and the second one dominates at higher PA concentrations. Experimental values of IL loading by PA and water in systems with ionic liquid [C14C6C6C6P][BTMPP] in equilibrium were successfully correlated by a model developed in our previous work. Good fit of data confirms a more general validity of this model. Stability constants of PA-IL complexes calculated by the model indicate that the affinity of PA to IL is lower compared to butyric acid but higher compared to lactic acid. Although PA-IL bonds are much weaker than butyric acid-IL bonds, the viscosity of equilibrium organic phases after PA extraction by IL is almost the same as in phases after butyric acid extraction.