Abstract: According to the invention, a pre-conditioning method of operating an autoclave during a pre-conditioning stage of a sterilisation cycle wherein the autoclave has a chamber having an external wall, a supply of steam, a heater to heat the external wall of the chamber and a vacuum pump, which comprises the steps of: applying a first vacuum pulse in the chamber; heating the chamber; supplying steam to the chamber at a pressure above atmospheric pressure for a holding period which is sufficiently long for the contents of the chamber to be heated; and applying a second vacuum pulse in the chamber; has been found to be effective as a pre-conditioning stage for a sterilisation cycle for sterilising a porous load.