A procedure for determining the size of inorganic polyphosphates of chain lengths up to about 750 is described. It involves reducing the size with polyphosphate glucokinase to a chain length that can be accurately determined by polyacrylamide gel electrophoresis. This measurement along with determination of the glucose-6-P formed and the total phosphate of the original polyphosphate permits calculation of the chain length. The accuracy of this method has been demonstrated by comparison with other reliable procedures. Thus far, it is the only method available for sizing long chain polyphosphates with nmol quantities.