The design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers. For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous gamma-Al(2)O(3), a widely used catalyst and catalytic support, transforms to non-porous alpha-Al(2)O(3) at approximately 1,100 degrees C (refs 7-10). Doping with La raises the transformation temperature to approximately 1,250 degrees C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds. Here, we use direct imaging by aberration-corrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and first-principles calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to alpha-Al(2)O(3). The results provide the first guidelines for the choice of dopants to prevent thermal degradation of catalysts and other porous materials.