Raman and IR spectroscopy of GeOn, with n=1,2,3,4, isolated in solid Ar were used for the elucidation of the corresponding molecular structures. The experiments were complemented by extensive ab initio calculations at different theoretical levels, in the case of Ge2O2 additionally by normal-coordinate analysis. The assignment of a planar, cyclic D-2h symmetric structure to Ge2O2, made formerly on the basis of IR spectroscopic investigations, was confirmed. It was possible to determine all vibrational frequencies of the molecule. The IR and Raman spectra of Ge3O3 allow for the unambiguous assignment of a highly symmetric D-3h ring structure for this molecule, as it has been proposed by other authors. In contrast we find that the previously assumed ring structure for Ge4O4 is incompatible with the measured IR and Raman spectra, as well as with the results of ab initio calculations. Instead a heterocubanelike structure, that is found for the heavier homologus molecules of Sn and Pb, is adopted in this case.