The composition of phosphorus-32 labeled polyphosphoric acid (32PPA) prepared from, (i) H, '"PO, and (82% w/w) P,O, and, (ii) by incorporation of radioactivity from H, '2P0, into unlabeled polyphosphoric acid (PPA) was examined and found to be the same. The identity of these condensed phosphorate mixtures, as established, argues strongly in favor of a rapid and complete equilibration and provides direct evidence in support of Van Wazer's reorganization theory of condensed phosphates. No evidence for metaphosphates or label exchange in an aqueous environment was found. "PPA was found to exist in metastabIe equilibrium with '"PPA as a crystalline solid of different composition. Agreement of hydrotytic rate data from the mixtures of condensed phosphates with literature values for individual species suggests that the hydro1ysis of the pyro-and tripoIyphosphate species proceeds independently in the presence of other homologs The rate of hydrolysis of the condensed phosphates (at pH5.0 and 65 "C) was found to be proportional to chain length, increasing about four-foId between pyro-and hexapoly-phosphate. The same correlation is also manifest in the adenosine 5' poiyphosphates. A slight enhancement in the rate of hydrolysis of adenosine diphosphate, increasingly more pronounced in the case of adenosine tri-and tetra-phosphate, was also noted. This effect is attributed to the presence of the adenosine moiety-The most interesting aspect of the rate data is to be found in the similarity of the rates of hydro1ysis of the adenosine 5' polyphosphates, reIative to their inorganic poiyphosphate counterparts. The significance of these findings is discussed and the viewpoint is presented that the properties of condensed phosphates are compatible with the requisite characteristics that must be predicated of any in s&f phosphoryl precursor in mitochondrial phosphorylation.