Silicon diodes irradiated with 1 MeV neutrons show relaxation behaviour, which results from deep levels that cause the material to become semi-insulating. The signature of these levels has been determined using the powerful technique of deep level transient spectroscopy. In particular, two deep levels are found not to anneal out with increases in fluence. It is possible to relate the activity of one of the levels to the relaxation behaviour of the diodes. The suggestion is that the relaxation behaviour of radiation-damaged silicon diodes is a consequence of the activity of those radiation-induced deep levels that are situated to act as generation-recombination centres. (C) 2002 Elsevier Science B.V. All rights reserved.