Ion-beam mixing of thin marker layers in amorphous silicon and germanium was studied using irradiations with Ze ions at temperatures of 34 K and 77 K. The marker species, ion energies and doses were: in silicon, markers of Ge and Pt irradiated with 200 keV Xe up to 2.7 × 1016 ions cm-2; and in germanium, markers of Al and Si bombarded with 295 keV Xe up to 1.63 × 1016 ions cm-2. In silicon, Pt markers were found to broaden at about the same rate at 34 K and 77 K; and the rate of broadening was similar to that found by other workers when expressed as an efficiency of mixing, i.e. when dependence on ion dose and deposited energy was factored out. However, a Ge marker irradiated at 34 K did not broaden from its original thickness. In germanium, markers of both Al and Si were mixed by irradiation at 34 K, but at 77 K only the Al marker broadened; the Si marker did not. The broadening of the markers is ascribed to ballistic mixing, while the cases where no broadening occurred are explicable if diffusion, by a defect mechanism, transported displaced marker atoms back to traps near their original sites. © 1983.