Both rat liver and Escherichia coli rRNA in 0.1m-sodium chloride were titrated with acid or alkali over the range pH3–7 at approx. 0°C. rRNA did not bind acid reversibly and hysteresis was observed, i.e. the plot of acid bound to rRNA against pH had the form of a loop showing that the amount of acid bound at a particular pH depended on the direction of the titration. Although the boundary curves were reproducibly followed on titration from pH7 to 3 and from pH3 to 7, points within the loop were `scanned', e.g. by titration from pH7 to a point in the range pH3–4 followed by titration with alkali to pH7. It is inferred that the `lag' in the release of certain bound protons is at least 1 pH unit, that at least about 9–15% of the titratable groups (adenine and cytosine residues) that are involved in this process and that the free energy dissipated in completing a cycle is approx. 4.2kJ/mol (1kcal/mol) of nucleotide involved in hysteresis. The interpretation of the `scanning' curves was illustrated by means of a cycle of possible changes in the conformation of a hypothetical nucleotide sequence that allows formation of poly(A)·poly(AαH+)-like regions in acidic solutions. It is also inferred that the extent of `hysteresis' might depend on the primary nucleotide sequence of rRNA as well as on secondary structure.