1. Two species of double-helical RNA isolated from mycelium of Penicillium chrysogenum were titrated with acid at 25°C and 95°C (solvent 0.1m-sodium phosphate buffer). At 25°C denaturation occurred at about pH3. At 95°C in the denatured form cytosine residues titrated as a simple monobasic acid of pK3.9 compared with pK≃2.5 for the native form at 25°C. 2. On thermal denaturation in neutral and acidic solutions one species of RNA (38% rG·rC) ‘melted’ in three distinct stages, equivalent to a mixture of three species, namely one of about 25% rG·rC, another of about 33% rG·rC and a third of about 46% rG·rC: the relative proportions were 0.25:0.35:0.40. 3. On thermal denaturation in acidic solutions the increase in the fraction of ionized cytosine residues concomitant with the ‘melting’ of rG·rC base pair also affects the spectrum especially at 280nm and serves to enhance the contribution of rG·rC base pairs at this wavelength. The increment in ε(P) at 280nm on ‘melting’ an rG·rC base pair approaches 53501·mol−1·cm−1 depending on pH, compared with 33501·mol−1·cm−1 at pH7. In contrast ε(P) at 280nm is scarcely affected by ‘melting’ rA·rU base pairs or by the protonization of adenine residues. 4. Changes in the spectrum of Escherichia coli rRNA on denaturation in acidic solutions were studied to yield the mole fractions of rA·rU and rG·rC base pairs ‘melting’ at particular pH values.