X-Ray diffraction measurements on two forms of dodecasodium myo-inositol hexaorthophosphate, obtained from aqueous methanol have shown them to have the formula C6H6O24Na12P6,xH2O with x= 33 for stable needles [form (A)] and x= 35 for efflorescent rhombic prisms [form (B)]. They can be converted to the compound with x= 38 [form (C)], first prepared from aqueous ethanol, by seeding the aqueous methanol solution with a crystal of form (C). Forms (A) and (B) are both monoclinic, with space group P21/n and Z= 4; the cell dimensions are form (A): a= 14·78, b= 32·14, c= 12·12, β= 102·0°; and form (B): a= 26·95, b= 20·3, c= 11·77, β= 111°.