The influence of nickel, yttrium and La-rich mischmetal additions on the stability and the nature of the crystalline phases formed during continuous heating of amorphous Mg–Ni–Y–MM alloys has been evaluated. The stability of the amorphous matrix is increased with increasing the content of the different alloying elements, especially yttrium and rare earth additions. All alloys exhibit at least one atomic rearrangement in their amorphous structure prior to crystallisation, which can be rationalised in terms of the development of medium range order zones of crystalline phases, mainly Mg17RE2 and Mg6Ni. Once Mg17RE2 crystallisation starts, the surrounding amorphous regions become enriched in nickel and yttrium, stabilising the remnant amorphous phase at higher temperatures. The composition of the remnant amorphous phase determines the nature of the phases crystallised in subsequent transformations.