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The effects of different pouring temperatures on the reaction behaviour of the ceramic shell with the molten Mg-9%Al-1%Zn-0.4%Mn (all are in wt-%) (AZ91) alloy during investment casting were studied. The scanning electron microscopy (SEM) micrographs with energy dispersive spectroscopy (EDS) analysis illustrated a complex interaction behaviour of the alloy with the mould components at various pouring temperatures. The MgO + MgAl2O4 compounds were formed on the surface of investment cast alloy at the pouring temperature of 600°C. With increasing the temperature to 650°C, the MgO compounds with the Al2O3 particles were observed at the interface of metal/mould. The Al2O3 particles were the main compound on the surface of alloy at the pouring temperature of 700°C. The formation of Al2O3 particles instead of the MgO compounds on the surface was related to the increase of magnesium evaporation by enhancement of the pouring temperature and also diffusion of Al2O3 particles from the ceramic shell to the surface of the alloy.