Organophosphorous compounds have been widely used in inorganic analysis for the extraction and separation of inorganic acids or metal species. Since these compounds can form hydrogen bonds to proton donors, they can also be used for the extraction of acidic organic compounds. Therefore, the reactive extraction of propionic acid using tri-n-octylphosphine oxide (TOPO) in hexane was studied. Equilibrium and kinetics experiments were performed. The extraction of propionic acid using n-heptane, light liquid paraffin, heavy liquid paraffin and hexane was studied and hexane was found to be most suitable diluent. The equilibrium complexation constant for the propionic acid-TOPO complex was determined to be 0.702 m(3)/kmol. The extraction was found to be first order in propionic acid and first order in TOPO with the overall rate constant as 46.91 (m(3)/kmol)(2)/s.