New magnesium-based bulk metallic glasses have been synthesized in the system Mg.,Cu, Gdz with cone-shaped copper mould casting. Compositions have been prepared with critical diameters up to 9 mm. X-ray diffraction patterns and DSC traces have shown that a composition Mg58.5Cu30.5Gd11 shows higher 7, and a large Delta T. The substitution of gadolinium by yttrium in this formula has been studied in the same conditions, and samples with critical diameters up to 9 mm have been prepared. The glass forming ability of these compounds has been discussed