Time-resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane, Me₃SiH. The reaction was studied in the gas-phase at 10 Torr total pressure in SF₆ bath gas, at five temperatures in the range 295–528 K. The second order rate constants fitted the Arrhenius equation: log(k/cm³ molecule^-1 s^-1)=(-12.15±0.06)+(11.58±0.44 kJ mol^-1)/RTln10. Experiments at other pressures showed that these rate constants were unaffected by pressure. GC evidence supports the formation of trimethylsilylgermane as product. The data are consistent with the involvement of an H-bridged complex as intermediate, through an encounter controlled first step. Comparisons show that alkyl substituents in the substrate molecule activate Si–H bonds more selectively than Ge–H bonds towards insertion by GeH₂. GeH₂ is a more selective, but less reactive, “heavy methylene’' than SiH₂. Reasons for this behaviour are discussed.