The new compound BaAl 2 Ge 2 exists in two modifications, α (low-temperature phase; space group Pnma, α-BaCu 2 S 2 type) and β (high-temperature phase; space group I4/mmm, BaZn 2 P 2 type, a variant of the ThCr 2 Si 2 type). The reversible, reconstructive α↔β phase transition is modelled with the help of topological descriptors (PNS), as a change from the layered β-phase to the α-phase with a 3D network, upon an increase of the puckering of the layers and the appearance of new bonds between them. The nature of the bonding in both modifications is approached by the electron localization function (ELF). The distribution of its maxima is correlated with the topological information. The physical properties of the compounds are determined and can be understood from electronic structure calculations within the tight-binding LMTO approach.