This thesis explores the thermal conditioning of three different legacy nuclear wastes requiring additional treatment. For the first waste stream (encapsulated Magnox drums showing evidence of internal corrosion) it was found that thermal conditioning passivates the reactants responsible for the underlying corrosion reactions, by oxidising both Magnox and uranium. At high waste loadings, the main constraint is forsterite crystallisation, which occurs at high Mg concentrations in the glass. Thermal conditioning offers a slight volume reduction compared to cementation. The resulting thermally conditioned waste form has a high short-term chemical durability with an increased durability observed when leached in saturated Ca(OH)2 solution (this result was also observed for the other two waste streams). For the second waste stream, clinoptilolite, thermal conditioning at low melting temperatures, as low as 900 oC, was demonstrated. The main benefit of these low melting glass formulations is the retention of Cs during processing (between 92% and 99%). Increasing the molar ratio of Na2O to B2O3 has a significant impact on glass structure and glass dissolution, depolymerising the network, decreasing NLNa and NLB slightly, but significantly increasing NLSi, due to the higher pH of the leachate. However chemical durability of these glasses was low, performing worse than HLW glass. For the third waste stream, co-conditioning of sand / clinoptilolite from SIXEP, with sludge waste, was found to significantly decrease waste volumes (by a factor of 6 compared to cementation). Similar to the encapsulated Magnox drums, the main constraint on glass formulation is Mg concentration, with forsterite crystallising at ~29 mol% of MgO. It was demonstrated that, for this compositional range, Mg behaves as a modifier and in the short-term forsterite serves to significantly increase the durability of the overall waste form. Overall, it was demonstrated that thermal conditioning offers many benefits and should be given strong consideration as a conditioning strategy for some of the U.K.'s legacy nuclear wastes.