Diffusion coefficients (D-H and D-D) of hydrogen (H) and deuterium (D) in beta-titanium (Ti) and beta-Ti3Al have been obtained from the rates of gaseous H and D absorption by samples in the temperature ranges of 1173 to 1473 K for beta-Ti and 1423 to 1523 K for beta-Ti3Al. Activation energies for diffusion (epsilon(d)) are found to be 0.26 eV in beta-Ti and 0.39 eV in beta-Ti3Al, which are smaller than those corresponding to alpha-Ti and alpha-Ti3Al. Ratios D-D/D-H, in beta-Ti and beta-Ti3Al are found to be larger than the classical value, 1/root 2, and those in alpha-Ti and alpha-Ti3Al. The observed smaller epsilon(d) in beta-Ti than in alpha-Ti is due to the difference between the body-centered cubic structure of beta-Ti and the hexagonal close-packed structure of alpha-Ti and the observed larger D-D/D-H in beta-Ti than in alpha-Ti due to phonon assisted jumps of diffusing H atoms in beta-Ti. The observed smaller epsilon(d) in beta-Ti3Al than in alpha-Ti3Al can be explained by the body-centered cubic structure of beta-Ti3Al and the random distribution of aluminum atoms in it, and the larger D-D/D-H in beta-Ti3Al than in alpha-Ti, alpha-Ti3Al, and beta-Ti by the presence of aluminum atoms as well as by the phonon-assisted H-atom jumps.