[en] Highlights: • Sealing process of anodic alumina films was studied by advanced SEM, TEM and XPS. • In boiling water, boehmite precipitates as small flakes due to (100)-oriented growth. • As pore walls dissolve, boehmite flakes increase, forming an intertwined structure. • A three-layered sealing film with different densities of flaky boehmite is formed. • Neither preferential pore mouth closure nor uniform pore narrowing was identified. The mechanism of the hot water sealing of anodic alumina films was studied using electron microscopy and compositional analysis. The films dissolve in boiling water and boehmite precipitates as small flakes due to (100)-oriented growth with 3-nm average thickness and evenly dispersing in the pores and the surface. The quantity and size of flakes increase with increasing sealing time forming a densely intertwined boehmite structure. Eventually, a three-layered structure is formed, which is composed of an outer vertically grown coarse flaky layer, an intermediate dense flaky layer with 2:1 thickness ratio, and a porous layer filled with dense flaky boehmite.