The acidity constants of the (N7)H+ sites of inosylyl(3'-> 5')inosine (Ipl(-)) were estimated and those of its (N1)H sites were measured by potentiometric pH titrations in aqueous solution (25 degrees C; l = 0.1 M, NaNO3). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg2+, CO2+, Ni2+, Zn2+, or Cd2+ (= M2+) and (Ipl - H)(2-) and, in the case of Mg2+, also of (Ipl - 2H)(3-). The stability constants of the M(Ipl)(+) complexes were estimated. The acidity constants of H(inosine)+ and the stability constants of the M(Ino)(2+) and M(Ino - H)(+) complexes were taken from the literature. The comparison of these and related data allows the conclusion that, in the M(Ipl - H) species, chelates are formed; most likely they are preferably of an N7/N7 type. or the metal ions CO2+, Ni2+, Zn2+, or Cd2+, the formation degrees of the chelates are on the order of 60-80%; no chelates could be detected for the Mg(Ipl - H) complexes. It is noteworthy that the (N1)H deprotonation, which leads to the M(Ipl - H) species, occurs in all M(Ipl)(+) complexes in the physiological pH range of about 7.5 or even below.