Preferential grain etching is a problem occurring on aluminum products during alkaline etching prior to anodizing, especially if the etching bath contains small amounts of dissolved Zn. The visually uneven, "grainy" surface is caused by different etch rates for different grains, giving a terracelike surface topography. Polished samples of an AlMgSi alloy with different amounts of Zn were etched in NaOH solution. Grain orientations in selected areas were determined by electron backscatter diffraction. The low-Zn samples did not show preferential grain etching. On the higher-Zn alloys, grains with surface planes close to {111} were found to etch at a higher rate than those with other orientations. Secondary ion mass spectrometry depth profiling and imaging, combined with white light interferometry analysis, showed a clear correlation between the deep lying grains and a surface enriched in Zn, suggesting that Zn enrichment by alkaline etching occurred by selective dissolution of aluminum rather than a dissolution-redeposition mechanism. Zn-enriched grains become more active than the other grains as a result of a decrease in the rate of the water reduction reaction. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3496068] All rights reserved.