Corrosion behavior of as-cast and heat treated Mg-Zn-Zr-Y alloy was studied in 1 g/l NaCl solution. The as-cast alloy was the least resistant to pitting corrosion. Its Mg-Zn solid solution matrix was the structural constituent responsible for corrosion susceptibility due to the presence of Zn-lean regions. Homogenization of the Zn distribution within the matrix significantly increased the corrosion resistance of the alloy. Grain boundaries, which contain intermetallic phases, were resistant to corrosion attack, thus impeding corrosion propagation.