Trialkyl (Cyanex 923) and trioctylphosphine oxides (Cyanex 921) dild. in toluene, octane or in low-aromatic naphtha b. 222-234 degrees C (Exxsol D 220/230) were studied as extractants of propionic (I), crotonic (II) and oxalic (III) acids from 0.01-0.1 M solns. at 20-70 degrees C. The extd. acids were stripped with water or aq. NaOH. Extn. and stripping isotherms were detd. I and II could be fully stripped with 4M NaOH from the loaded org. phase. Multistage extn. and stripping were also examd. Equil. was always established in 5 min. At lower acid concns. and temps., the yields of extn. were higher. Extractant/acid 1:1, 2:1 and 3:2 complexes were formed in the org, phase. The apparent enthalpy and entropy data for the complex formation with Cyanex 921 dild. in octane at 20 degrees C were: I, -21.11, -40.6; II, -24.19, -44.4; III, -27.47 kJ/mol, -81.4 J/mol-K, resp.