Metallic glass is a class of materials which have a disordered structure of atoms, due to this, glasses lack grains and grain boundaries, which are present in their crystalline counterparts. Metallic glasses have many interesting properties worth investigating, such as high corrosion resistance or high mechanical strength. However, metallic glasses are metastable and will therefore crystallise if heated above the crystallisation temperature. MoSiZr alloys have been studied and to gain knowledge of how the composition affects the crystallisation temperature, which enables further improvement of thermal stability. Crystallisation temperatures of the MoSiZr alloys were investigated by heat treatments in vacuum and ex-situ X-ray diffraction and X-ray reflectivity analysis. The highest thermal stability of the alloys was exhibited by M48Si48Zr4, Mo43Si50Zr7, Mo50Si40Zr10 and Mo45Si43Zr12, they remained amorphous after heat treatment at 1073 K. The resulting crystalline phases are Mo3Si, Mo5Si3 and ZrO2. Oxidation of Zr in the alloys is present only when the Zr content is at least 10 at%, crystallisation is otherwise mainly driven by formation of Mo3Si. Further improvement of the thermal stability is possible by introducing new alloying elements at the cost of those that promote crystallisation. Keeping the content of Zr below 10 at% is of great importance to prevent oxidation.