Dry machining of five commercial magnesium alloys with aluminium content in the range 2-9 % has been investigated by turning tests. At high cutting speeds, workpiece material may build up on the flank sides of carbide tools (FBU). The critical speed for FBU formation is dependent on the presence of β-phase (Mg17Al12). The amount of β-phase increases with increasing aluminium content, and depends on the temper condition of the alloy. The formation of FBU can be eliminated by the use of a suitable cutting fluid or a PCD tool.