The activation energy of tritium diffusion in intermetallic 6LiAl crystals (103.8±9.5 kJ/mol) determined in the temperature range from 699 to 886 K was fairly large compared with that of hydrogen diffusion in Al (40–60 kJ/mol) in which hydrogen would diffuse in an interstitial-diffusion mechanism. The tritium diffusion in 6LiAl crystals was explained by a vacancy-diffusion mechanism, roles of vacancies and anti-structure atoms being considered.