The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation time (as usual), yielding a chemical diffusivity that is unequivocally that of the component oxygen. In a water-activity gradient in a fixed oxygen activity atmosphere, on the other hand, the conductivity relaxation appears quite unusual, exhibiting an extremum after an initial transient. The conductivity relaxation upon hydration or oxidation, in general, is quantitatively analyzed in terms of two apparent chemical diffusivities for component oxygen and hydrogen, respectively. The inner workings of hydration is discussed, and the as-evaluated chemical diffusivities are reported and compared with the conventional chemical diffusivity of water.