Ethanol affects unequally the thermal stability of DNA and RNA. It stabilizes RNA, while destabilizing DNA. The variation of the relative viscosity (η/η₀) of [poly(dA–dT)]₂ with temperature unveils transitions close to the respective denaturation temperature, calculated spectrophotometrically and calorimetrically. From the raw data densities and speeds of sound, the volumetric observables were calculated. In all cases studied, a change in sign from low to high ethanol content occurred for both partial molar volume (ϕV) and partial molar adiabatic compressibility (ϕK_S). The minima, close to 10%, should correspond to the highest solvation and the maxima, close to 30%, to the lowest solvation. For 40–50% ethanol, the solvation increases again. The complex structure of ethanol–water, for which changes are observed in regions close to such critical concentrations, justifies the observed behaviour. The variation of ϕV and ϕK_S was sharper for RNA compared with respect to DNA, indicating that the solvation sequence is poly(rA)·poly(rU) < ct-DNA < [poly(dA–dT)]₂.