Abstract The stability constants of the 1:1 complexes formed between Mg2 +, Ca2 +, Sr2 +, Ba2 +, Mn2 +, Co2 +, Ni2 +, Cu2 +, Zn 2+ or Cd 2+ and orotidinate 5′-monophosphate (OMP3-) were determined by potentio-metric pH titrations in aqueous solution (I =0.1 M, NaNO3; 25°C). In addition to the stability constants of these M(OMP)− complexes, for several cases also the corresponding acidity constants for the release of the proton from the H(N-3) site were calculated; i.e., the formation of M(OMP-H)2- complexes was quantified. On the basis of recent measurements for simple phosphate monoesters [R-MP2-; R is a noncoordinating residue; S.S. Massoud and H. Sigel, Inorg. Chem., 27, 1447-1453 (1988)], evidence is provided that the somewhat increased stability of all the mentioned M(OMP)− complexes is mainly the result of a charge effect of the carboxylate group (in position 6 of OMP3-) and not of a direct participation in complex formation; i.e., there are no indications for the formation of significant amounts of macrochelates involving the phosphate and the carboxylate groups. This is different for the M(OMP-H)2- complexes of Co 2+ , Ni2+ and Cd2+: in these cases significant amounts of macrochelates form; i.e., the metal ion is not only coordinated to the phosphate group but also (in part) to the ionized −(N-3) site, which is placed in the neighbourhood of the phosphate residue in the dominating syn conformation of this nucleotide. For the metal ions Mg2 +, Ca2+, Sr2 +, Ba2+ and Mn2 +, which have in general a rather low affinity for N binding sites, no evidence for the formation of macrochelates is detected. In addition, the stability constants of the ternary Cu(Arm)(OMP)− complexes, where Arm = 2,2′-bipyridyl or 1,10-phenanthroline, were determined by potentiometric pH titrations. Evaluation of the stability data shows that an equilibrium betweeen an ‘open’ isomer and a Cu(Arm)(OMP)− species with an intramolecular stack exists; the formation degree of these aromatic ring stacks reaches about 40 percent. Overall it is quite evident that OMP3- is a versatile ligand with remarkable properties which may be utilized by nature in recognition reactions during the intricate metabolic processes in which this nucleotide is involved.