Propionic acid is widely utilized in different chemical applications and industries such as plastic, coating, agricultural, chemical and perfume industries. However, propionic acid occurs in the waste streams of these industries and is produced in aqueous solutions by fermentation processes. Thus, removal of propionic acid from both waste streams and production medium is an important topic. In this experimental and optimization study, the reactive extraction of propionic acid from its aqueous solutions was investigated. Imidazolium-based ionic liquids namely, 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF6]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][Tf2N]), were utilized as diluents, and tributyl phosphate (TBP) was utilized as an extractant. The effect of different factors like initial acid concentration (5-10%, w/w), initial TBP concentration in ionic liquids (0-3 mol L-1), and aqueous/organic phase ratio (0.5-1.5) on the extraction efficiency was investigated. The optimal conditions were determined by using central composite design (CCD) based on response surface methodology (RSM). The optimization work showed that within the investigated parameters, the most effective parameter was the initial TBP concentration in ionic liquids. The optimum extraction conditions were obtained as initial propionic acid concentration of 5% (w/w), TBP concentration in ionic liquids of 3 mol L-1 and phase ratio of 0.5. Under these conditions, the experimental extraction efficiencies were found to be 87.56% and 88.16% for [HMIM][PF6] and [HMIM][Tf2N], respectively. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.