The protonic conductivity of CsHSO4 and CsDSO4 was measured with in situ isotopic exchange using van der Pauw and 2-electrode geometries for the high- and low-temperature phases, respectively. The proton/deuteron conductivity ratio of around 2.3 in the high temperature phase is a result mainly of a 0.02 eV higher activation energy of deuteron conductivity, assigned to a difference in zero-point energy. The low temperature phases exhibited no large isotope effects and hence no indications of proton tunneling.