Synthesis and characterization of the two new macrocyclic ligands 1,4,7,16,19,22-hexaza-10,13,25,28-tetraoxacyclotriacontane (L1) and 1,4,7,16,19,22,25-heptaza-10,13,28,31-tetraoxacyclotritriacontane (L2) are reported. Both of them contain two polyamine units separated by two eight-atom dioxa bridges. The basicity constants and the formation constants of their complexes with Cu(II) and Zn(II) have been determined by potentiometric measurements in aqueous solution (298.1 K, I = 0.15 mol dm(-3)). Both L1 and L2 form mono- and dinuclear complexes in aqueous solution. In the [ML1](2+) and [ML2](2+) (M = Zn2+ or Cu2+) complexes, the metal ion is coordinated by only one polyamine moiety. The binuclear complexes show a marked tendency to form mono- and dihydroxo species in aqueous solution. The high values of the constants for the addition of the first hydroxide group to the dinuclear species suggest that in the [M(2)L1(OH)](3+) and [M(2)L2(OH)](3+) species the OH- group bridges the two metal centers. This thermodynamic result is confirmed by the crystal structure of the [Zn(2)L1(mu-OH)](ClO4)(3) . 0.5EtOH compound (a) (space group Pbca, a = 14.289(4) Angstrom, b = 15.617(6) Angstrom, c 33.862(8) Angstrom, V = 7556(4), Z = 8, R = 0.0612, wR(2) = 0.1780) which shows the hydroxide ion bridging the two Zn(II) cations (Zn ... Zn distance 3.5 A). In the crystal structure of [Cu(2)L1(NCS)(4)] (b) (space group P2(1)/a, a 8.550(6) Angstrom, b = 17.854(6) Angstrom, c 12.090(3) Angstrom, beta = 102.23(4)degrees V = 1804(2), Z = 2, R = 0.0520, wR(2) = 0.1618) each metal ion is coordinated by one triamine unit and by two isothiocyanate ions in a distorted square pyramidal arrangement. Finally, in the crystal structure of [Cu(2)L1(NCS)(4)] Na2S2 (c) (space group P-21/a, a = 8.492(3) Angstrom, b = 17.740(4) Angstrom, c = 13.016(3) Angstrom, beta = 94.54(3)degrees, V = 1954.7(9), Z = 2, R = 0.1101, wR(2) = 0.3105) the two metal ions exhibit a coordination environment almost equal. to that found for (b), but the [Cu(2)L1(NCS)(4)] units are joined by NCS---Na+---(S-S)(2-) ---Na+ interactions giving rise to a tridimensional polymeric structure.