Magnetic after-effect measurements of H, D and T in both dilute Ni alloys and in a face-centred cubic Co alloy at low temperatures have revealed significant isotope effects of the jump frequencies nu , the pre-exponential factors nu ₀ and the activation energies Q. The isotope effects are much more pronounced between H and D than between D and T. Identical jump frequencies and activation parameters ( nu ₀, Q) were determined for D and T in the Co alloy. In Ni and its alloys between 120K and 160K, the order of jump frequencies was found to be nu ^H>> nu ^T> nu ^D, i.e. T was found to be more mobile than D. A comparison with the results of H, D and T diffusion experiments in Ni at higher temperatures shows that for H and D high- and low-temperature ranges must be distinguished. The transition from high- to low-temperature diffusion is a gradual one and begins for H below room temperature and for D below 170K. No transition is observed for T. The diffusion of H isotopes is interpreted as being a competitive process of over-the-barrier jumps and tunnelling transitions. For the diffusion of H, D and T in Ni, from 120K to 160K, the following effective activation parameters were determined: Q^H approximately=0.350 eV, Q^D approximately=0.395 eV, Q^T approximately=0.400 eV, nu ₀^H approximately=8.9*10¹⁰ s^-1, nu ₀^D approximately=7.1*10¹¹ s^-1, nu ₀^T approximately=1.4*10¹² s^-1.