In this study, propionic acid removal from aqueous solutions by tributyl phosphate (TBP) in a room-temperature ionic liquid was investigated. A room-temperature ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide [HMIM][Tf2N], was used as a diluent. TBP was uti-lized as a reactant. The relationship between the extraction efficiency and various process parameters like initial propionic acid concentration, initial TBP concentration in [HMIM][Tf2N], and water/organic phase ratio were analysed by using Box-Behnken design which is one of the response surface methodology (RSM). The optimum extraction conditions for the reactive extraction of propionic acid with [HMIM] [Tf2N] were detected as initial propionic acid concentration of 5.429 % (w/w), TBP concentration in [HMIM][Tf2N] of 2.854 mol.L-1 and phase ratio of 0.558.(c) 2022 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.