Base specificity in the interaction of ethidium with double stranded synthetic RNA homopolymers has been studied by means of spectroscopic (UV-visible absorption and fluorescence), microcalorimetric and dilatometric techniques. The results show a strong base specificity in this interaction, the association constant with poly A:poly U being more than three order of magnitude higher than with poly O:poly C. The interaction is mainly enthalpy driven, the differences in affinity being essentially entropic in origin. These evidences along with the dilatometric data suggest that the observed base specificity may arise from the different extent of water release upon intercalation.