It has been generally accepted for some time (1) that phosphorylation processes occur concomitantly with oxidation in the electron transport system via pyridine nucleotide, flavoprotein, and cytochrome c to oxygen; indeed in recent years direct experiments by Lehninger and coworkers have demonstrated that phosphorylation accompanies the oxidation of reduced diphosphopyridine nucleotide in mitochondrial preparations (2). However, there have been no experiments which give an insight into the nature of the interaction of the electron transport system with phosphate or any part of the phosphorylating system. The present paper deals with a new approach to this problem with Ols-labeled phosphate. The rationale of the method depends upon the fact that the phosphate group does not necessarily proceed intact through a sequence of phosphorylation, transphosphorylation, and dephosphorylation reactions, but may lose one or more of its original oxygen atoms. Thus, by labeling the oxygen of the phosphate group, it may be possible to follow the path of the phosphate group through a series of reactions in which the phosphorus leaves no trace. If inorganic phosphate labeled with 01* is taken up in organic linkage by the formation of a carbon-oxygen bond as in phosphorylase reactions (3) and in the glyceraldehyde phosphate dehydrogenase reaction: the oxygen bridging the carbon and phosphorus becomes labeled with O’*. Should the organic phosphate now be cleaved by the rupture of the phosphorus-oxygen bond as in phosphatase reactions (3), the organic moiety remaining would contain 018. Moreover, if inorganic phosphate were formed in such a reaction, one of the four labeled oxygens would have been replaced by normal oxygen from the water. This opens up two possibilities, (1) the identification of phosphorylated intermediates by the presence of Oi* in the dephosphorylated products and (2) the detection of reactions otherwise unobservable by following the loss of Oi* from inorganic phosphate.