Complexation of technetium with tetraalanine yields an unstable complex that converts to a monooxotechnetium(V) complex of cyclic tetraalanine. The structure of the latter was determined by single-crystal X-ray analysis. In this negatively charged complex, the technetium atom is coordinated to one oxygen and four amide nitrogen atoms. The more polar complex that is initially formed is assumed to be an oxotechnetium(V) complex of tetraalanine, with the oxotechnetium moiety coordinated to three amide nitrogen atoms and the amine nitrogen atom. In this complex, the carboxyl group would be forced in the vicinity of the free amine group, thereby facilitating amide formation and cyclization. Cyclization of tetra-L-alanine occurs on both no carrier added (technetium-99m) and carrier added (technetium-99) scale.