The macroscopic diffusion coefficient and low-frequency vibrations of sodium atoms adsorbed on a Pt(111) surface have been measured by helium atom scattering along the [11(2) over bar] azimuth at a low coverage of Theta (Na) = 0.043, The results show chat the sodium atoms diffuse rapidly with rates from D = 3.7 x 10(-5) cm(2)/s to D = 2.3 x 10(-4) cm(2)/s in the temperature range from T-s = 100 K to T-s = 350 K, indicating a low diffusion barrier (E-d < 50 meV). The parallel vibrational mode frequency of h omega = 5.3 meV is comparable to the value for Na/Cu(001) of h omega = 5.85 meV for the same coverage and temperature.