The rate of decomposition of aqueous solutions of 5-phosphoribosyl-α-1-pyrophosphate (PRPP) is greatly enhanced by the presence of certain divalent ions. High levels of Mg 2 + were shown to increase the rate of phosphate removal from PRPP by factors greater than 100-fold. The combined action of Mg 2 + and a nitrogenous compound (imidazole was employed in this study) increases the degradation rate constants by up to 1000-fold. The degradation of PRPP in neutral solutions follows two main paths, a pyrophosphate hydrolysis reaction yielding ribose 5-phosphate and an internal β-phosphate cleavage yielding 5-phosphoribosyl-1,2 cyclic phosphate. The catalytic effect of Mg 2 + on PRPP degradation appears to suggest a connection between the observed rate enhancements and the dimagnesium form of PRPP. Rate enhancing effects of Mg 2 + are greatest at pH values (i.e. pH>7) which promote strong phosphate-Mg 2 + interaction.