Ni(II) and Co(II) coordination with the macrocyclic ligand 1,4-dioxa-7,10,13-triazacyclopentadecane (L) has been investigated in solution (Co(II) and Ni(II)) and in the solid state (Ni(II)). Complexation equilibria have been studied by means of potentiometric titrations in 0.15 mol dm(-3) NaCl, at 298.1 +/-0.1 K, evidencing the formation of [ML](2+), [MLOH](+), and [ML(OH)(2)] (M = Co, Ni) species. Under aerobic conditions, the mononuclear Co(II) complex binds dioxygen, forming the dibridged [Co2L2(O-2)OH](3+). This is the unique oxygenated species formed. The importance of the bridging OH- anion for the formation of the oxygenated complex has been evidenced by competitive coordination of SCN-. Structural information has been obtained by resolving the crystal structure of the [NiLCl][NiL(H2O)](ClO4)(3) compound. The structure contains [NiLCl](+) and [NiL(H2O)](2+) cations in which the metal ions have octahedral coordination environments determined by five ligand donor atoms and one Cl-/H2O.