The reaction with 2-propanol of purines and pyrimidines, induced photochemically with light of lambda greater than 300 nm and di-tert-butyl peroxide as an initiator, was applied to a variety of adenosine-, guanosine-, and uridine-containing ribonucleotides in order to determine the rules which govern the reactivity of the heterocyclic bases of nucleotides. The reactivity of the purine moieties was found to depend on the conformation of the appropriate nucleotide (anti or syn) and on the site of binding of the phosphate group to the ribose moiety. Adenosine moieties (assuming an anti conformation) blocked at their 3'-hydroxyl reacted faster than those blocked at their 5'-hydroxyl. The reactivity of the guanosine moieties (tending to assume a syn conformation) was independent of the site of binding of the phosphate. The uridine moieties of the various nucleotides exhibited a wide range of reactivity. A correlation between the reactivity of the uridines and their involvement in stacking interactions with next- and second-neighboring purines could be made. Thus, the uridine moieties of U-U-U, G-U, U-G, A-U-A, and A-U-G were reactive, while those of A-U and A-U-U were unreactive. The relative reactivity of uridine moieties of nucleotides can, therefore, be used as a measure of the extent of pyrimidine-purine stacking and vice versa.