A flow-through cell for hydrothermal phase transformation studies by in situ and time-resolved neutron diffraction has been designed and constructed. The cell has a large internal volume of 320 ml and can operate at temperatures up to 573 K under autogenous vapor pressures (ca 8.5 × 106 Pa). The fluid flow is driven by a thermosyphon, which is achieved by the proper design of temperature difference around the closed loop. The main body of the cell is made of stainless steel (316 type), but the sample compartment is constructed from non-scattering Ti-Zr alloy. The cell has been successfully commissioned on Australias new high-intensity powder diffractometer WOMBAT at the Australian Nuclear Science and Technology Organization, using two simple phase transformation reactions from KAlSi2O6 (leucite) to NaAlSi2O6·H2O (analcime) and then back from NaAlSi2O6·H2O to KAlSi 2O6 as examples. The demonstration proved that the cell is an excellent tool for probing hydrothermal crystallization. By collecting diffraction data every 5 min, it was clearly seen that KAlSi2O 6 was progressively transformed to NaAlSi2O 6·H2O in a sodium chloride solution, and the produced NaAlSi2O6·H2O was progressively transformed back to KAlSi2O6 in a potassium carbonate solution.