This chapter has presented the basic methods involved in the use of 31P NMR to study positional isotope exchange in enzyme-catalyzed reactions involving phosphorus-containing substrates. The method is straightforward but requires synthesis of specific 18O isotopically labeled substrates at the site of bond cleavage. Analysis of the kinetic consequences of the PIX reaction depends on the nature of the enzymatic reaction and the number of other substrates involved in the kinetic reaction mechanism. This may be simple or formidable. From the examples described in this article one can appreciate that PIX experiments, when combined with other kinetic methods, can in favorable cases unravel many (if not all) of the rate constants in an enzyme-catalyzed reaction.