Mg-Zn metallic glasses, characterised by low crystallisation temperatures, are useful alloys for investigating the room temperature stability of the amorphous metallic state. In this paper the structural stability of Mg-Zn metallic glasses is discussed. As-quenched ribbons and the same ribbons after storage for six years at room temperature were investigated through differential scanning calorimetry (DSC), X-ray diffraction and optical microscopy. The fully amorphous structure was found only for Mg65Zn35 composition. An Mg68Zn32 alloy exhibited an amorphous structure with precipitated Mg on the ribbon surface. Mg75Zn25, Mg72Zn28 and Mg70Zn30 showed a mixture of crystalline phases with a decreasing amount of amorphous phase on the Mg-rich side of the peritectic. The structure of the crystalline phases and the quantity of the remaining amorphous phase was determined. Thermal analysis of aged samples indicated two values of activation energy for crystallisation-lower for the Mg-rich side of the peritectic (including the peritectic composition) and higher for Zn-rich alloys. Additionally, for Zn-rich alloys thermal effects related to the structural relaxation of the amorphous phase were observed.