Abstract: In chemical production processes having flammable substances in a reactor (1), the reactor is rendered inert by feeding an inert gas, while headspace gas is removed from the headspace of the reactor. This inert-rendering must also be maintained if the reactor must be opened during the production process. Even when inert gas locks (3) are used, the inert-rendering can only be reliably maintained in this case with a complex instrumental determination of the oxygen content in the reactor. This expense can be avoided by establishing and maintaining a certain difference between inert gas flowrate and head-space gas flowrate removed in a defined manner, which difference is specific to the particular production factors and is determined in advance.