Abstract A new application of He atom scattering is introduced. It is shown that the quasielastic scattering of low-energy He atoms can be used to study two-dimensional diffusion at surfaces on an atomic scale. Measurements are presented for the self-diffusion along the [11̄0] azimuth at the Pb(110) surface at a temperature of 521 K. Other experiments have shown the Pb(110) surface to be partially disordered at this temperature, as a result of the onset of a surface melting transition. From the dependence of the width of the quasielastic energy distribution, of scattered He atoms, on the initial and final angles, both the diffusion mechanism and the diffusion coefficient are derived. The results show that the diffusion coefficient at the surface along [1 1 0], at 521 K, exceeds the value for bulk-liquid Pb at the bulk melting point, T pb m = 600.7 K. The diffusive motion can be described in terms of jumps with a continuous distribution of jump lengths and an average jump length of ∼ 4.4 Å.