The devitrification products of two simulated high level radioactive waste (HLW) glasses have been investigated. Magnox waste glass contained RuO2 as its primary crystal phase, Pd-Te inclusions, a Cr-, Fe- and Ni-rich spinel phase, a Si- and lanthanide-rich phase and also a Zr-rich phase which incorporated Ce and Gd. Upon heat treatment the glass developed a zektzerite-type phase, CeO2, a strontium molybdate (containing Nd and La) as well as a Si-rich phase. 75/25 glass, comprising a blend of reprocessing waste derived from UO2 and Magnox fuels, contained RuO2 as its primary crystalline phase, CeO2 and a Si- and Ru-rich phase. Heat treatment of this glass resulted in the growth of the CeO2 crystals, the development of a strontium molybdate (containing Nd and La) and a Si- and lanthanide-containing phase. A sodium lanthanum molybdate with a powellite-type structure formed on the surface of both glasses after heat treatment in air.