The Cs-133 and deuteron NMR measurements of the superionic transition in CsDSO4 showed that the mechanism of protonic conductivity involves thermally-activated hopping of deuterons over well-defined lattice sites. The present 2D NMR study of O-D...O deuteron interbond motion provided evidence that slow deuteron exchange between two differently oriented hydro en bonds in the unit cell of CsDSO4 already takes place at room temperature, thus indicating precursor effects of the superionic transition far below the transition temperature T-0 = 412 K.