Thermal helium scattering is used to characterize the deposition of silver atoms on Pd (100) in the coverage range 0 to 10% of a monolayer, at surface temperature between 80 and 160 K, the latter being the threshold of adatom mobility. The attenuation of the specular helium intensity as a function of coverage cannot be interpreted by a statistical distribution which excludes any mobility of the deposited adatoms. The data are well simulated by a model implying that an adatom is not stable if it is separated from another adparticle by only one adsorption site, but it would rather move and form a dimer or enlarge a cluster. This behavior is confirmed by Monte Carlo simulations. The cross section of the silver adatom for diffuse helium scattering is measured to be 109 AA/sup 2/.