Magnesium oxide (MgO) boards are relatively new cladding materials on the Australian construction market. They are chosen primarily for their fire resisting properties and are fast replacing traditional cladding materials in newer buildings. Recent reports of water staining on MgO boards is thought to be due to the effects of relative humidity promoting corrosion of metallic fasteners. Different commercially available MgO board samples were investigated by following their behaviour in a controlled relative humidity environment and when in contact with different metallic fasteners. It is observed that water absorption is different for different products and that staining varied with the composition of the metallic fasteners. Staining and water absorption was not a problem at a low relative humidity (RH) of 75% but was evident at a high humidity of 97% for all but one of the MgO board samples. The concentration of chloride is implicated and has its origin in the binder material. These results emphasise the need for standard characterisation of commercially available MgO boards for their use in high humidity environments.