Separation of carboxylic acids from their aqueous solutions by reactive extraction using a newly developed deep eutectic solvent (DES) was investigated. DESs were prepared using trioctylphosphine oxide (TOPO, T) and menthol (M) binary mixtures prepared in different mole ratios. H-1 NMR (nuclear magnetic resonance spectroscopy), P-31 NMR, and differential scanning calorimetry (DSC) analyses were performed to characterize the solvents that were obtained. The physical properties of the DES formed were determined by measuring density, viscosity, and refractive index values. Extraction was carried out using the DESs with organic acids (formic, acetic, propionic, glycolic, lactic, malic, and citric acid) containing different numbers of hydroxyl and carboxyl groups. Extraction efficiency (E %) and distribution coefficient (D) values were calculated to evaluate the obtained results. Response surface methodology was used to investigate the effects of the experimental conditions on reactive extraction yield and to obtain model equations of the acids.