The diffusion of K, Rb, and Cs has been studied by implantation of low energy ions into a-Si and using high-resolution Rutherford backscattering for measurement of the resulting profiles. The diffusion is many orders of magnitude slower than in c-Si. Trapping and detrapping effects have been observed. Trap depth and average distance have been estimated. a-Si appears as an inhomogeneous solid consisting of a disordered bulk with voids embedded