The ultraviolet and circular dichroism spectra of authentic captopril have been obtained, since the reported literature data are inconsistent with those obtained on highly purified material. The UV absorption spectrum consists of a single band maximum at 200 nm, while the CD spectrum consists of a single negative peak located at 210 nm. The CD spectrum of captopril and its other three diastereomers can be explained largely in terms of a summation of the chirality of its individual components, (S)-proline and (2S)-3-mercapto-2-methylpropionic acid.