Rare earth (RE) can produce excellent precipitation hardening in Mg alloys. However, when forming a solid solution, it also deteriorates formability, a problem that can usually be overcome by raising deformation temperature. Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy. As the temperature reached 500 ℃, the formability decreased drastically, leading to severe intergranular fracture under only 0.5% strain. This was caused by failure of grain boundaries, which are weakened by segregated interfacial compounds.