Diffusion of Si adsorbates deposited on a Si(001) surface is investigated by reflection electron microscopy. At temperatures up to about 600 °C, the diffused lengthxof the adsorbates is proportional tot1/4before the critical timetc,wheretis the heating time. Att<tc,xis determined by a length at which atoms collide with others on the surface. Att>tc,the diffused lengthxis proportional tot1/2,wherexis determined by thermal diffusion of the adsorbates. ©1997 American Institute of Physics.