Gas-phase rate constants for the title reactions have been obtained by laser flash photolysis at 297 K, by use of photoprecursors, 3,4-dimethyl-1-germacyclopent-3-ene for GeH2 and pentamethyldigermane for GeMe2. The values obtained were (k (cm(3) molecule 1 s(-1))): (2.38 +/- 0.11) x 10(-10) for GeH2 (2.26 +/- 0.10) x 10(-13) for GeMe2. These results show that the insertion reaction of GeMe2 is 1050 times slower than that of GeH2 into the Ge-H bonds of Me2GeH2. This is explained in terms of a general mechanism involving an intermediate H-bridged complex, applicable to both silylene and germylene insertions. For the GeMe2, insertion, reactants are in equilibrium with the complex, which rearranges to the product in the rate controlling step. (C) 2002 Published by Elsevier Science B.V.