The microstructure and mechanical properties of a Mg-1.48Gd-1.13Y-0.16Zr (at.%) alloy in the as-cast, solution-treated, peak-aged and over-aged conditions have been investigated by a combination of thermodynamic calculations and experimental approaches. It is shown that both the Mg-24(Gd,Y)(5) and cuboid-shaped Mg-5(Gd,Y) phases exist in the as-cast sample, which is in good agreement with the Scheil solidification model. The former is dissolved during solution treatment, while the latter persists and coarsens. Subsequent artificial ageing results in the formation of metastable beta' precipitates within the alpha-Mg matrix and along the grain boundaries. The peak-aged alloy exhibits maximum ultimate tensile strength and tensile yield strength of 370 and 277 MPa, respectively, at room temperature. Moreover, the strengths decrease gently from room temperature to 250 A degrees C with a gradual increase of elongation. The strengthening contributions to the yield strength are quantitatively evaluated from individual strengthening mechanisms by using measured microstructural parameters. The modelled yield strengths are compared with the experimental results and a reasonable agreement is reached.