adenosine 5'-triphosphate (ADPH 4* and ATPHr=,+ in strongly acidic medium) visrbIe circular dichroism (d-d transitions of Co27 and ultraviolet circular dichroisrn (adenine transitions) have proven to be very sensitive to structural changes. Drastic variation of spectra as a function of pH and concentration enabled us to show the existence of various species, to state their stoichiometry and eventuahy, their self-association. With ATPHz2-, CD. results are in agreement with recent N.M.R. results. With ligands bearing three negative charges, complexes (1 metal:2 nucleotides), are formed in which bases of the two nucleotides of the molecule are self-associated_ With ADP3, the visrble CD. spectrum of this complex is intense and hides the spectra of the com-plexes formed with other protonated species of ADP; this self-associated complex is detected up to a lower limit of 5 X lo4 M concentration. With ATPH3-, a complex exhibiting the same characteristics as the one with ADP3-is formed but in about twenty times less amount which explains why it was not detected by potentiometry. With 0.1 M ATP*, dirneric (or polymeric) complexes, of 1 r2 and 1:l stoichiometry are observed. With 0.01 M A*, lrl monomeric ad 211 dimeric (or poIymeric) complexes are ds tected. The interactions between Mn 2* ions and ADP cr ATP have been studied by CD. on the W range. The same species as with Co2+ ions have been found but the 1:2 compiex formation with ADP3-was shown to occur to a lesser extent and was not observed below a 10e2 M ADP concentration.