The stability constants, log K M(NTP) M, were determined (or taken from earlier work by the author) for many of the 1:1 complexes between the nucleoside 5′-triphosphates (= NTP), ATP 4−, ITP 4−, GTP 4−, CTP 4−, UTP 4− or TTP 4−, and the bivalent metal ions (= M 2+) Ca 2+, Mg 2+, Mn 2+, Co 2+, Ni 2+, Cu 2+ or Zn 2+ ( I = 0.1, NaClO 4; 25°). In agreement with earlier work and contrary to recent data and conclusions it is clear that the stability of the complex for a given metal ion is to a first approximation determined solely by the coordinating ability of the triphosphate chain; the influence of the nucleic base moiety on complex stability is small, though for those metal ions with a pronounced coordination tendency towards nitrogen donors, i.e. Co 2+, Ni 2+, Cu 2+ and Zn 2+, the stability constants of the purine 5′-triphosphate complexes are on average about 0.4 log unit larger than that of the pyrimidine 5′-triphosphate complexes. This observation indicates a metal ion-base interaction in these M 2+-purine 5′-triphosphate complexes. In addition, the stability constants log K M(Bipy)(NTP) M(Bipy) for ternary 2,2′-bipyridyl-M 2+-nucleoside 5′-triphosphate complexes with the 3 d-metal ions mentioned have been determined. For most of these systems the difference, Δ log K M = log K M(NTP) M − log K M(Bipy)(NTP) M(Bipy), is positive, i.e. the following equilibrium is shifted towards its right hand side: M(Bipy) 2+ + M(NTP) 2− ⇌ M(Bipy)(NTP) 2− + M 2+. In other words, these mixed-ligand complexes are more stable than would be expected on a statistical basis. The stability constants for some of the protonated binary and ternary complexes, M(HNTP) − and M(Bipy)(HNTP) −, are also given. The location of the proton in these complexes is discussed, as such complexes can be formed not only with ATP and CTP, which have a rather basic nucleic base moiety, but also with ITP, GTP, UTP and TTP. In the protonated complexes with these latter nucleotides the proton must be bound to the γ-phosphate group, while with ATP and CTP an equilibrium between the two isomers, one with the proton at the base moiety and the other with the proton at the γ-group, is possible. The formation of the complex species in dependence on pH is shown in several figures.