Crystal structure of disodium adenosine 5’-monophosphate dodecahydrate (Na(2)AMP.12H(2)O) was determined. Crystal data are C222(1), a = 6.8768(6), b = 19.287(3), c = 41.099(6) Angstrom, and Z = 8. Bases are stacked to make a columnar structure along the a axis. There exist water layers parallel to the ab and ac planes, and molecular columns are isolated by the water layers. Sodium ions are coordinated by six water molecules, and columnar structure of hydrated sodium ions runs parallel to the molecular column. Phase transition of Na(2)AMP undergoes between 0 to 90% relative humidity by four steps. Coupled with increase or decrease of the number of water molecules, cell parameters b and c seem to change.