Myoinositol hexaphosphate has been prepared by Posternak's method and purified from lower phosphate esters by ion-exchange chromatography. The product behaved differently from the natural hexaphosphate when used as a substrate for a plant phytase preparation. By modifying Posternak's original method it has been found possible to prepare a synthetic hexaphosphate which is hydrolysed at the same rate as its natural counterpart. The hexaphosphates of scyllo-, neo-and D-inositol have been prepared by the modified method and have been compared as substrates in phytase hydrolysis experiments.