Following the pathway transfer process, the C-A cycle can be either Ga-hydride-centered (region c, Fig. ) or Ga-alkyl-centered (region d, Fig. ), and the two cycles differ by terminating the oligomer chain growth through β-hydride elimination or by directly transferring hydrogen to an ethylene molecule. In our Ga/SiO2 system, we report that only the less-constrained, threecoordinated Ga 3+ site is responsible for the oligomerization reactivity through the Ga-alkyl centered C-A cycle, of which the free energy landscape is significantly more favorable than the competing proton transfer and the hydride-centered C-A mechanisms. The selective ethylene dimerization observed in experiments is enabled by an unreported transition state of the ethyleneassisted β-hydride transfer step to terminate lengthening of the carbon backbone, and the activation energy is much lower than that of the ethylene insertion step. Finally, a detailed microkinetic analysis is carried out, based on the DFT results, which establishes that the ethylene insertion step is rate-limiting. In aggregate, these insights establish a comprehensive mechanistic understanding of how amorphous silica-supported main group single sites catalyze olefin oligomerization.