The high selectivity and efficiency of micellar electrokinetic chromatography with a borax-sodium dodecylsulfate (SDS) or meglumin-SDS buffer make possible the rapid separation of hydroperoxy and hydroxy fatty acids and the non-oxidised unsaturated fatty acids from which they are derived. Nearly all the isomers of the hydroperoxides and hydroxy fatty acids derived from oleic, linoleic, alpha- and gamma-linolenic and arachidonic acids can be determined both qualitatively and quantitatively within ca. 10 min. The system has as many as 1 x 10(6) theoretical plates, and the detection limits with UV diode array detection at 195 or 234 nm are in the micromolar range.