Time-resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with C₂D₄. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295–554 K. The second-order rate constants gave the Arrhenius equation: log(k^∞/cm³ molecule⁻¹ s⁻¹) = (−10.76 ± 0.06) + (5.79 ± 0.46 kJ mol⁻¹)/RT ln10Pressure variation measurements over the range 1–100 Torr (SF₆) at 295, 406 and 554 K showed the rate constants to be pressure independent within experimental error. Comparison with the reaction of GeH₂ + C₂H₄ studied earlier, which was pressure dependent, gave large inverse isotope effects which increased with increasing temperature and decreasing pressure. The data supports a mechanism, involving the reversible isomerisation of initially formed germirane-d₄ to ethylgermylene-d₄ and is consistent with previously carried out thermochemical and ab initio calculations of the energy surface.