Modified group-IV(1 00) surfaces have been successfully utilized as a template for growing crystalline oxide films, e.g., BaO. Knowledge of structural and electronic properties of surface reconstructions on group-IV(100) substrates can be used for the control of abruptness and electric properties of oxide/group-IV interfaces. In the present study, reversible Ba/Ge(100) c(4 x 4) ↔ (1 x 2) phase transition, governed by the adsorption of oxygen at room temperature, as well as structural properties of both surface phases have been studied in detail by low -energy electron diffraction, scanning tunneling microscopy, and core-level photoemission using the synchrotron radiation. An atomic model is proposed for the c(4 x 4) reconstruction, and it is shown how this structure is modified upon the interaction with oxygen. Finally, the useful properties of O-containing (1 x 2) reconstruction, which allow one to optimize the growth of crystalline oxide films, are discussed.