The thermal stability and creep behaviour of MgNi2Y1CeMM1 and MgNi3Y1.5CeMM1.5 alloys have been investigated at 523 and 623 K. Both alloys were processed by a powder metallurgy route involving rapid solidification of powders, cold isostatic pressing and extrusion at 673 K. The microstructure of both alloys was studied in the as-extrusion condition and after thermal treatments at 523 and 623 K up to 500 h. Mechanical characterization was carried out by hardness measurements in samples annealed at 523 and 623 K and stepwise stress compressive creep tests at these temperatures. Results have shown long-term stability of the microstructure after annealing at 523 K in both alloys and hardness values remained constant along all treatment. After treatment at 623 K, however, the microstructure of both alloys remained relatively stable and a decrease in hardness was noticed after 6 h. Creep behaviour of both alloys was very similar. The stress dependences of the creep rate can be well described by modified Garofalo sinh relationship. Activation energies for creep, practically twice of the activation enthalpy of lattice diffusion for pure magnesium (135 kJ mol-1), have been obtained. Therefore, no direct evidence of the controlling role of diffusion processes can be obtained from these experiments.