The corrosion of binary Mg-Zn, Mg-Sr, and ternary Mg-Zn-Sr alloys was studied in conjunction with microhardness and microstructural characterization. The study was designed to isolate the effect of Sr on the corrosion of Mg, which has not been previously reported, and to classify its interaction with Zn in Mg alloys. The results reveal that the addition of Sr to Mg leads to localized corrosion, even at relatively low alloying levels (~0.05 wt%). The addition of Zn has no significant effect on the corrosion of Mg over a relatively wide range of concentrations (up to 2 wt%). In ternary Mg-Zn-Sr alloys, the impact of Sr on corrosion is mitigated by the presence of Zn. The magnitude of the effect of thermal processing on corrosion depends mainly on the alloy composition and was most profound in the Mg-Sr alloys.