A fire suppression system comprising: at least one high pressure gas source containing an inert gas, wherein the inert gas is nitrogen, helium, argon, carbon dioxide, or mixtures thereof; at least one low pressure gas source containing a second gas; a distribution network connected with the at least one high pressure gas source and the at least one low pressure gas source to distribute the inert gas and the second gas, the distribution network including flow control devices configured to control flow of the inert gas and the second gas; and a controller in communication with the distribution network, the controller configured to: initially release the inert gas in response to a fire threat to reduce an oxygen concentration at the fire threat below a preset oxygen concentration threshold, and release the second gas to increase a second gas concentration at the fire threat above a preset second gas concentration threshold while the oxygen concentration is below the preset oxygen concentration threshold; characterised in that: the second gas is an organic halide gas; and the controller is configured to release the organic halide gas in response to a minimum oxygen concentration at the fire threat, wherein the minimum oxygen concentration is indicated by a continuous decrease in an instant or detected oxygen concentration provided by a detection system, followed by a change to an increase in the instant or detected oxygen concentration. at least one high pressure gas source containing an inert gas, wherein the inert gas is nitrogen, helium, argon, carbon dioxide, or mixtures thereof; at least one low pressure gas source containing a second gas; a distribution network connected with the at least one high pressure gas source and the at least one low pressure gas source to distribute the inert gas and the second gas, the distribution network including flow control devices configured to control flow of the inert gas and the second gas; and a controller in communication with the distribution network, the controller configured to: initially release the inert gas in response to a fire threat to reduce an oxygen concentration at the fire threat below a preset oxygen concentration threshold, and release the second gas to increase a second gas concentration at the fire threat above a preset second gas concentration threshold while the oxygen concentration is below the preset oxygen concentration threshold; initially release the inert gas in response to a fire threat to reduce an oxygen concentration at the fire threat below a preset oxygen concentration threshold, and release the second gas to increase a second gas concentration at the fire threat above a preset second gas concentration threshold while the oxygen concentration is below the preset oxygen concentration threshold; the second gas is an organic halide gas; and the controller is configured to release the organic halide gas in response to a minimum oxygen concentration at the fire threat, wherein the minimum oxygen concentration is indicated by a continuous decrease in an instant or detected oxygen concentration provided by a detection system, followed by a change to an increase in the instant or detected oxygen concentration.