The surfaces of the gamma and eta forms of alumina are well known to differ significantly in Lewis acidity. Surface reconstructions observed on gamma- and eta -alumina are also qualitatively different, despite the close similarity of the bulk structures. Here we demonstrate through first-principles calculations that subtle differences in the bulk point defect distribution between these two forms of alumina give rise to the major differences in the mode of surface reconstruction and correlate with the different levels of Lewis acidity.