The influence of 2, 4 and 6wt.% yttrium additions on the mechanical properties and structure of magnesium-based alloys was investigated. The precipitation sequence was studied at 150, 200 and 225 degreesC by hardness measurements, microstructure investigations and X-ray phase analysis. The hardening of the alloys resulted from the appearance of orthorhombic beta" transition phase precipitating homogeneously throughout the alloy at 150 degreesC. The precipitation of the beta' and beta (Mg24Y5) phases was observed at dislocations and grain boundaries after prolonged ageing, which both do not contribute to the strengthening effect. At 225 C, there is only a little increase in hardness. (C) 2002 Elsevier Science B.V. All rights reserved.