The following compounds were investigated by ion-exchange or potentiometric methods for complex formation in the presence of divalent metal ions: uracil-5-carboxylic acid, uracil-6-carboxylic acid, 5-nitro-orotic acid, 2-ethylthio-iso-orotic acid, 2-thio-iso-orotic acid, adenosine-3′-phosphate (3′-AMP), adenosine-5′-phosphate (5′-AMP) and α- d-glucose-1′-phosphate (1′-GP). Radiochemical cation-exchange experiments indicated no complexing of Zn(II) ions with uracil-6-carboxylic acid, while mononuclear species were found for the remaining pyrimidine complexes of Zn(II), Co(II) and Mn(II), in the pH range studied. Formation constants calculated by cation exchange methods were found to be in good agreement with titration values. Potentiometric studies of adenosine-3′-phosphate indicated that the predominant species was the normal mononuclear complex (MA), but that the existence of a protonated complex (MHA) does exist. Equilibrium constants were calculated for MHA complexes of 3′-AMP and Ni(II), Co(II), Mn(II) ions; formation constants were determined for MA complexes of Zn(II), Co(II) and Mn(II) using ion-exchange and titration methods. For 3′-AMP, the stability of the MA complex followed the order, Zn > Co > Mn. Formation constants calculated for the MA mononuclear 5′-AMP complexes indicated the trend in complex stability, Ni > Co > Mn. An iterative computer programme was written for normal and protonated complexes for dibasic acids and divalent metal ions. Complexation studies of α- d-glucose-1′-phosphate supported the views of previous investigators that the adenosine residue does not play a major role in metal complexing. Formation constants were calculated for mononuclear 1'GP complexes of Zn(II), Co(II) and Mn(II). The relative order of stability for Co(II) and Mn(II) complexes of the phosphate ligands studied was found to follow the order, 5′-AMP > 1′-GP > 3′AMP.