A device (1) for sterilization in production of packages (8) is adapted for sterilization with a gaseous sterilizing agent kept in the gaseous phase throughout the sterilization process. The device (1) comprises a heating zone (2), a sterilization zone (3) and a venting zone (4). It further comprises an ambient temperature sensor (27) for sensing the ambient temperature outside the device (1), a concentration meter (29) for measuring the concentration of sterilizing agent in the sterilization zone (3) and a first control unit for controlling the amount of sterilizing agent introduced in the sterilization zone (3) based on the temperature measured by the ambient temperature sensor (27) and the concentration measured by the concentration meter (29). A method of sterilizing packages (8) in production of the packages (8) is also disclosed.