The formation process of magnesium potassium phosphate hexahydrate was studied by monitoring the pH development of the MgO-KH PO -H O system. It is found that a typical potentiometric curve goes through several rises and drops, forming two local peaks and two local valleys, before reaching a plateau at pH of approximately 10.5. The characteristic points of the curve are identified and then utilized to distinguish the different reaction stages during the formation of the hexahydrate. By comparing the potentiometric curves of the ternary systems with different molar ratios, and from X-ray diffractometry analysis of the precipitates extracted at different stages, the formation of the hexahydrate most likely proceeds stepwise through two crystalline intermediates, namely MgHPO .7H O and Mg KH(PO ) .15H O. The thermal behavior of the formation process of magnesium potassium phosphate hexahydrate, including the formation and transformation among the intermediates and final product, is further verified by thermodynamic calculation. Finally, the morphologies of the different crystalline phases of the ternary system are examined by a scanning electron microscope (SEM). © 2012 American Society of Civil Engineers.