Excitation spectroscopy of the F-7(0) --> D-5(0) transition of Eu3+ is used to detect and characterize a kinetic intermediate in the formation of a complex between Eu3+ and the macrocyclic ligand dtpa-dien (1,4,7-tris(carboxymethyl)9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane). Both the long-lived intermediate and the final product, [Eu(Hdtpa-dien)(H2O)](+), are formed immediately upon mixing the components, as evidenced by separate peaks in the excitation spectrum. The transformation of the intermediate to the final product, monitored by excitation spectroscopy, occurs by both proton-assisted and non-proton-assisted pathways. It is proposed that the intermediate represents a "blind alley" in the pathway to a final nine-coordinate tricapped trigonal prismatic product. The intermediate with an "up, down, up" configuration of carboxylate arms must undergo some decoordination to form the final complex. Molecular mechanics calculations and excited state lifetimes suggest that the intermediate is eight-coordinate with one coordinated water molecule. The stability constant of the final complex is found to be log K = 14.11 +/- 0.05.