γ-alumina is known to transform to θ-alumina and finally to α-alumina upon thermal treatment with a catastrophic loss of porosity and catalytic activity. First-principles calculations were performed toinvestigate the atomic scale mechanism of the γ- to θ-alumina transformation. The transformation pathways between the two different forms have been mapped out and identified as a sequence of Al cation migrations. Different possible Al migration paths may be responsible for the experimentally observed formation of domains and twins in θ-alumina. The estimated temperature dependence of the conversion rate is in excellent agreement with the experimental transformation temperature.