To extend knowledge of germanium-selenium double bonds. unimolecular reactions of HXGe=Se (X = H and F) have been investigated and compared with those of HXC=Se. by means of ab initio calculations including polarization functions and electron correlation. Five different reaction mechanisms are proposed: (A) 1.1-HX elimination, (B) 1.2-H shift, (C) 1.2-X shift, (D) H and XGeSe radical formation, and (E) X and HGeSe radical formation. Our theoretical findings suggest that HXGe = Se is kinetically stable with respect to the unimolecular reactions given above. Moreover. these comparisons emphasize that selenium is more reluctant to form double bonds with germanium than with carbon. We also report theoretical predictions of molecular parameters and vibrational IR spectra of HXGe=Se and its derivatives, which should be useful for further experimental observations.