Structures and harmonic vibrational frequencies for several simple transient germenes, H2Ge=CH2 (1), MeHGe=CH2 (2), Me2Ge=CH (3), FHGe=CH2 (4), and H2Ge=CHF (5), and their dimerization and transition state energies for the head-to-tail and head-to-head self-coupling have been calculated by nb initio HF3-21G and 6-311G(d,p) and density functional theory B3LYP/6-311G(d,p) methods. The effect of substituents on the Ge=C bond lengths and bond orders, as well as frequencies and force constants of the Ge=C stretch, is predicted to be relatively small in germenes 1-3 and be substantially stronger in the F-substituted molecules 4 and 5. Within the limits of the HF/3-21C method, the head-to-tail cyclodimerizations of all studied germenes 1-5 were found to be more exothermic than the head-to-head processes and to proceed with very low or zero barriers. This conclusion is confirmed for 1-4 by the higher level calculations with the 6-311G(d,p) basis set at HF and B3LYP levels, which in the case of 5 lends to the opposite result such as a substantial barrier for the head-to-tail dimerization and a lesser exothermicity of this process relative to tile head-to-head one. The comparison of the HF/3-21G data available for the whale series of similarly substituted silenes with those calculated for 1-5 at the same level of theory indicate somewhat easier dimerization of germenes with respect to their double-bonded silicon analogues.