We present a new method to determine surface diffusion constants using specular reflected thermal helium scattering. The specular reflected helium intensity is recorded in real time as a function of surface coverage of the adsorbate and surface temperature. The experiment is compared to a model calculation based on rate equations for the generation and destruction of adatoms and clusters. We find E/sub d1/=0.35+or-0.03 eV and nu /sub 01/=3*10/sup 9/s/sup -1/ as surface diffusion barrier and pre-exponential factor for the system Ag/Pd(100).