Abstract: Water mist for fire control and suppression has received considerable attention as one of potential methods for replacing halon. The effectiveness of water mist fire suppression systems (WMFSS) can also be further enhanced by additives. A series of experiments has been conducted to explore the CH₄/air fire suppression effectiveness by using water mist with metal chloride additives. The results show that the metal chloride can obviously improve fire suppression efficacy of water mist, which can be affected by the type and concentration of metal chloride. Six types of metal chloride as the additives were used in experiments. It is found that, with the same mass fraction, their fire suppression effectiveness is gradually enhanced in order of MgCl₂, CaCl₂, NaCl, KCl, MnCl₂ and FeCl₂. Because of three co-existing effects, there must be an optimal fire suppression effectiveness, which corresponds to the concentration matching of metal chloride with water mist.