The preparations and circular dichroism spectra of an optical isomer of cis-[Co(en)(NH,),Cl2]+, and some of the isomers of [Co(R-pn)(NH3)2Cl2]+ are reported. To facilitate the interpretation of the c.d, spectra, the absorption spectra of these complexes and a series of closely related complexes with the CoN4Cl2 chromophore were analysed to give the position of the tetragonal components of the 1A1g →1T1g cubic absorption bands. The absolute configurations of the isomer of cis-[Co(en)- (NH3)2Cl2]+ and of (+)-[Co(en)(NH3)2CO3]+, from which the dichloro compound was prepared, were determined by a comparison of the c.d. spectrum of the carbonato complex with that of D(+)-[Co(en)2CO3]+.