A study has been carried out on the effects of zinc and gadolinium concentration on the microstructure, properties and quench sensitivity of four alloys of similar composition to the commercial magnesium alloy Elektron 21. Increasing the gadolinium content of the alloys produced enhanced solid-solution strengthening which inproved the strength of the alloy in all heat treated conditions. Increasing the zinc content increased the quench sensitivity of the alloys. The quench rate from the solution treatment temperature had a significant affect on the precipitates formed during subsequent ageing.