Absolute rate constants have been obtained for the title reaction over the pressure range 1-100 Torr (in SF 6 bath gas) and the temperature range 293-415 K, by means of laser flash photolysis to generate and monitor germylene, GeH 2 . The reaction showed the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation:log(k ~ /cm 3 molecule - 1 s - 1 )=(-10.86+/-0.19)+(7.16+/-1.23kJmol - 1)/RTln10The parameters correspond to a fast reaction occurring at ca. 59% of the collision rate at room temperature. Collision efficiencies for GeH 2 π-additions are not much less than those of SiH 2 . RRKM (Rice, Ramsperger, Kassel, Marcus) modelling, based on a variational transition state consistent with the kinetics, gave a good fit to the data, with the value of E o =105 kJmol - 1 corresponding to ΔH o =122+/-12 kJmol - 1 for 2-methylgermirane decomposition. This result in conjunction with other work shows that C-methyl substitution lowers the thermal stability of germirane, and that germiranes are ca. 50-60 kJmol - 1 less stable to decomposition than their silirane counterparts.