Systems of two or more hydrogen-containing phases in solution equilibrium with hydrogen may not be in isotopic equilibrium. The resulting exchange of hydrogen isotopes between the phases is discussed with respect to both the kinetic behaviour and the final equilibrium state. The advantages of tracer experiments using tritium are discussed in terms of the experimental arrangement and the information attainable. It is shown that the equilibrium separation factor reflects the state of the dissolved hydrogen in different sorption states and that the exchange kinetics are affected by the state of the interphase. The effect of the total hydrogen pressure on the tracer exchange with tritium is discussed. The relationships developed are applied to the metal-hydrogen systems PdH n and Ti 0.4Mn 0.6H n . The results are supplemented by pressure-composition isotherms, neutron vibrational spectra, scanning electron micrographs, Auger electron spectra and Auger electron spectroscopy depth profiles obtained using argon ions.