Ge substrates of (100) orientation were irradiated with 1.0 MeV Ge ions at temperatures in the range from -180 o C to 500 o C. Pronounced swelling of the irradiated material up to 4000 Å, associated with the formation of a porous surface layer, was evident only for temperatures between -50 o C and 200 o C. The extent of swelling was found to be insensitive to temperature in this range and exhibited an approximately linear dose dependence for doses up to 1x10 1 7 Gecm - 2 . For temperatures outside this range, only sputtering effects were observed. The structure of the porous surface layer was examined by transmission electron microscopy. For samples irradiated at 22 o C, it was shown to have a density ~30% that of bulk Ge. These layers were further shown to be stable during subsequent annealing to 500 o C.