The self‐diffusion of tritium in artificially and naturally grown ice monocrystals, parallel and perpendicular to the optical axis, was studied between −2.5° and −35.9°C. The plane‐source solution to Fick's second law was used in treating the data. An anisotropy of ≈ 12% was observed. The activation energy was found to be 0.62 eV for all cases andD0≈ 10 cm2/sec. Based on the experimental data, it is concluded that the diffusion takes place by a vacancy mechanism, and that entire H2O molecules are diffusing, i.e., molecular diffusion occurs. Theoretical calculations using the atomic‐diffusion theory and Zener's theory forD0are in excellent agreement with the experimentally determined diffusion coefficient.