The content of free MgCl2 is one of the key indicators of deliquescence, dehalogenation and efflorescence for Magnesium Oxychloride Cement (MOC). A range of possible techniques for assessing the free MgCl2 content of MOC have been investigated. The techniques are based on mixing powdered samples with absolute ethyl alcohol and measuring the amount of MgCl2 passing into solution. The influence of temperature of solvent, extraction time, fineness, agitation and liquid to solid ratio (L/S) are discussed. The results of recovery tests ranged 96.37% or above. The method is successfully applied to leaching and detecting free MgCl2 in commercial magnesium oxychloride cement fireproof board.