This paper describes the synthesis, crystal structure, and aqueous solution behavior of a polymeric bismuth citrate compound (NH4)(12)[Bi12O8(cit(4-))(8)](H2O)(10) (1). Compound 1 was obtained from an aqueous solution containing commercially available bismuth citrate (BiC6H5O7) in the presence of ammonia, which crystallizes in the rhombohedral space group with a = b = 17.807(3) Angstrom, c = 31.596(6) Angstrom, V = 8684(4) Angstrom(3), and Z = 3. The structure solution and refinement based on 1203 reflections with I = 3 sigma(I) converged at R = 0.055 and R(w) = 0.062. The polymeric structure of compound 1 is attained by carboxylate bridges from citrates in an anionic dodecanuclear bismuth-oxo citrate cluster unit [Bi12O8(cit(4-))(8)](12-). The dodecanuclear unit with a symmetry of inversion can be composed of two hexanuclear [Bi6O4(cit(4-))(4)](6-) cluster units, each of which uses three carboxylate bridges from three crystallographically equivalent citrate ions to connect the units. A fourth citrate in the smaller cluster unit does not obey the 3 symmetry within one unit but does obey the symmetry in the crystal lattice by orientational disorder. The solution behavior of compound 1 has been investigated by using H-1 and C-13 NMR spectroscopy. In highly concentrated solutions, the Bi-12 clusters are still aggregated to some extent. However, the order of aggregation decreases with a decrease in the concentration, resulting mixtures of Bi-12, Bi-24, and (Bi-12)(n) species. In diluted solutions, even the Bi-12 unit (or the smaller unit Bi-6) undergoes hydrolytic decomposition to release free citrates. Furthermore, when an excess of free citrate is added to the diluted solution, the cluster species completely decomposes, resulting in the ligand exchange process, and all citrates become averaged on the NMR time scale. The solution behavior of compound 1 was compared with that of earlier reported bismuth citrate compounds, including commercially available colloidal bismuth subcitrate (CBS; a well-known ulcer-healing agent), leading to the conclusion that solid samples of CBS which are amorphous could be formed by aggregation of the dinuclear building block [(cit(4-))BiBi(cit(4-))](2-) through citrato bridges and hydrogen bonding.