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Coincidence doppler broadening (CDB) spectroscopy has been applied to study the precipitation process induced by aging in Mg&-1.0 wt.% Ca and Mg&-1.0 wt.% Ca&-1.0 wt.% Zn alloys. In addition positron lifetime experiments and microhardness measurements have been performed. A peak centered at 11.5 × 10−3m0c is found in the CDB ratio spectra of the alloys aged at 473 K. It is attributed to annihilations with the core electrons of Ca. The results indicate the formation of a particle dispersion that hardens the alloys. This dispersion is correlated with the appearance of the peak attributed to Ca atoms. Zn atoms in the Mg matrix inhibit the formation of quenched-in vacancies bound to Ca atoms in the aged ternary alloy producing the dispersion refinement.