Focusing on phosphate hydration, the two phosphate oxygen atoms O R and O S exhibit very distinct dynamics in RNA, which is not the case in DNA. The specific slowdown of O R hydration is not due to the presence of water molecules bridging successive phosphate groups, that have similar dynamics to that of non bridging hydration water at these sites. Instead, we showed, using our Extended Jump Model, that it is caused in RNA by both a larger excluded volume and a stronger initial H-bond next to O R . These two factors originate from the markedly different phosphate orientations in RNA, which result from the different double helical conformation of B-DNA and A-RNA. The same framework was used to rationalized the absence of slow sites in the RNA grooves.