Sterilization by gas can be effectively employed on materials, which are heat sensitive and moisture sensitive. The most frequently used gases for this purpose are ethylene oxide, propylene oxide, and formaldehyde.

The gases mentioned are highly flammable in mixture with air, so they are diluted with an inert gas such as carbon dioxide or a suitable fluorinated hydrocarbon. A special equipment resembling autoclave is used for this method of sterilization. The method of sterilization with gas is enhanced and the exposure time required is reduced by increasing the relative humidity of the system to about 60% and by increasing the exposure temperature to 50°C to 60°C.

The method of sterilization takes an exposure time of 4–16 hours. The mechanism of sterilization is by interfering with the metabolism of the bacterial cell. Ethylene oxide gas has good penetrating power, which makes it very useful for sterilization of catheters, needles and plastic disposable syringes in their final plastic packaging before the shipment. Heat-labile enzyme preparations and antibiotics can also be sterilized by this method.