We report the temperature dependence of the H2 and H8 purine ring proton resonances of oligoriboadenylates up to chain length 11, with or without a single guanosine residue a t the 5'-end, second position, or 3'-end. The results suggest the following generalizations: (1) Stacking of the bases in a right-handed single-stranded helix is more extensive in the interior of the chain than at the chain ends. (2) The tendency of the terminal base to unstack is greater at the 5'-end than a t the 3'-terminus. (3) G stacks more weakly than A, as evidenced by weak stacking of 3'-terminal G. Anomalies were also observed in the unstacking profile of G a t the second position in the chain, indicating a conformational anomaly such as looping out of G, thereby allowing adjacent A's to stack together, or adoption by G of some other alternative structure. (4) The results imply that the environment a t a given base is influenced by effects of longer range than nearest-or next-nearest-neighbor. Increasing ion condensation as chain length increases may be responsible for the slow approach of oligomer behavior to the properties of the high polymer.