A sterilization method comprising subjecting a sterilization chamber to an initial vacuum and then introducing steam to increase chamber pressure. The introduction of steam is interrupted and the chamber is brought into communication with a solution in a gaseous sterilant generator. The mixture of gaseous sterilant and chamber atmosphere are continuously circulated whereby equilibrium is established and the material to be sterilized is subject to constant temperature, pressure, sterilant concentration, and relative humidity for a period of time sufficient for sterilization. Also disclosed is an apparatus capable of performing the sterilization method comprising a sterilization chamber in combination with a gaseous sterilant generator. Formaldehyde is the preferred sterilizing agent.