DSC and in-situ neutron diffraction have been used in order to find the phases formed under various heating treatments in the vicinity of the glass transition for Mg-based bulk metallic glasses (BMG) The first phase to crystallize in the Mg(585)Cu(305)Y(11) alloy, when the sample is hold at T(g) + 20 K, is Mg(2)Cu and a nanocrystallized phase During continuous heating of the same sample, the main phases to crystallize are Mg(2)Cu, MgY and Cu(2)Y The alloy Mg(555)Cu(28.5)Ag(5)Y(11) has the best GFA parameters observed for Mg-based BMG (Delta T=54 degrees and gamma=0.415) and presents a single event for crystallization The phases formed by holding the sample at T(g) +20 K are the same than observed for the alloy without silver (Mg(2)Cu and nanophase) During continuous heating of this sample the same phases as previously observed appear but with Ag(3)Mg as extra phase. If ternary phases are present, they could correspond to tiny lines not refined, but they appear with a very low intensity Mg(2)Cu and Cu(2)Y produce the main intensity lines The appearance of nanocrystals during crystallization in the glass transition region could be at the origin of the plasticity observed in these alloys