Magnesium alloys containing IIIa transition metals, such as Sc, Y and Ho, respectively, were hydrogenated at 773 K and examined for microstructure, X-ray diffraction pattern, micro-Vickers hardness, and tensile properties at room and high temperatures. Results obtained are as follows : 1. The alloys, respectively, have been internally hydrided and have precipitated hydrides of the IIIa transition metals as small flake-like particles in the matrix and at grain boundaries, as well as twin boundaries. 2. The dispersed hydride particles do not necessarily contribute to further hardening of the alloys at room temperature and up to near 673 K. 3. However, the dispersed particles are very stable and seem to improve mechanical properties of the alloys above 673 K. 4. Presumed relationships of crystallographic coincidence between the matrix and hydrides have been obtained.