NMR study of polycrystalline caesium hydroxide monohydrate CsH3O 2 shows a fast protonic diffusion in its high temperature phases. From a direct measurement, the diffusion constant is D = 1.2 x 10-5 exp (-1700/T [K]) cm2 s-1. 1H relaxation data and 2H spectrum indicate a strong 2-d character for this diffusion which is shown to be of Grotthus-type : it takes place through threefold H3O+ rotations combined with H jumps along O-O. Two dimensionality comes from the layered structure and the diffusion mechanism.