The present invention relates to an immobilizing medium for the encapsulation of radioactive waste. The waste immobilizing medium has a sodium silicate based glass matrix in which there is contained radioactive waste wherein the waste comprises one or more inert metal components and one or more fission products. At least a portion of the inert metal components are dissolved in the glass matrix and increase its durability. As a result, the waste immobilising medium is highly durable and leach resistant and is suitable for long term storage of radioactive waste. The inert metal components preferably comprise iron, nickel and chromium.