Magnesium oxide (MgO) construction boards are currently being used to replace fibre cement boards in various construction applications. This include usage in external claddings where reports of hygroscopic degradation were linked to presence of residual chlorides originating from the manufacturing process. The industry has responded with new products that are being marketed as Magnesium Oxysulphate (MOS) boards that are still essentially MgO based but are manufactured from non-chloride precursor. In this paper the hygroscopic performance of the new MOS based MgO is compared with some other commercially Chloride containing samples. It is shown that MOS based samples exhibited lower water absorption comparable to fibre cement boards. No evidence of chloride in the new MOS based MgO boards was found by Energy Dispersive Spectroscopy (EDS). Small angle neutron scattering (SANS), of dry samples and hydrated samples using deuterated water and normal water showed that the better hygroscopic properties of MOS based MgO board was not related to structural morphological modification and are more likely due to the absence of residual chloride in the new generation MgO boards.