Laser-excited luminescence spectroscopy of the F-7(0) --> D-5(0) transition of Eu3+ reveals rapidly formed intermediate species when this ion is mixed with dota (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate). This intermediate converts to the final complex [Eu(dota)H2O](-), with rate constants, k(obs), under conditions of saturating excess metal ion, which increase from 7.2 x 10(-4) to 7.9 x 10(-2) s(-1) as the pH is raised from 3.78 to 5.81. The course of the reaction is followed by the increase in intensity of the excitation peak (579.77 nm) of the final product as it is formed. The intermediate (lambda(ex) = 579.20 nm) was determined to be the diprotonated species (EuH(2)dota(+)). The excited state luminescence lifetimes of this species measured in H2O and D2O solutions reveal 4.5 +/- 0.5 coordinated water molecules, consistent with a proposed structure in which the Eu3+ ion is coordinated to four ligand carboxylate oxygens well away from the four ligand nitrogens of the macrocycle, two of which are protonated. Molecular mechanics calculations suggest that five water molecules complete the coordination sphere.