The initial atmospheric corrosion of ZnAl2Mg2 coated steel in humid air with 80ppb SO2 was studied using in situ Infrared Reflection Absorption Spectroscopy, XRD and SEM.Corrosion products formed in SO2- and SO2 and NaCl containing environments on ZnAl2Mg2 (ZM) were dominated by Mg-containing sulphite and sulphates with sulphite formed mainly initially. The sulphite formation is connected with dissolution of the Mg-containing oxide layer present initially. This is followed by a localised corrosion process where Mg rich phase in the binary eutectic microstructure is dissolved anodically while the Zn-rich phase is the site for the cathodic oxygen reduction.