Surface exchange and diffusion coefficients of the protonic conductor Nd_5.5W_0.5Mo_0.5O_11.25−δ were obtained by means of conductivity relaxation experiments as a function of temperature, pO₂ and pH₂O. The two-fold non-monotonic behaviour in the hydration/dehydration relaxations allows proton and oxygen vacancy coefficients to be independently determined whereas the H/D isotopic effect was confirmed.