An HPLC method has been developed for the separation of four stereoisomers of ephedrine using precolumn derivatization with S(+)-1-(1-naphthyl)-ethyl isocyanate. The formed derivatives are subsequently separated on a normal-phase column and are detected at a UV wavelength of 220 nm. This method was used to quantitate the differences in the enantiomeric impurity of various samples of (+)-pseudoephedrine. The reported method can differentiate between samples of (+)-pseudoephedrine which differ in their enantiomeric impurity by as little as 0.02%. Possible racemization of (+)-pseudoephedrine in aqueous solutions was also studied. Samples of (+)-pseudoephedrine from various suppliers and, indeed, different lots from the same supplier, differed significantly in their degree of enantiomeric impurity.