The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C 2 H 2 island formation and adsorbed C 4 H 4 , which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C 2 H 2 diffusion is determined to be 43 ± 1 kJ/mol. The activation barrier for formation of the C 4 H 4 intermediate is found to be 84 ± 6 kJ/mol and the barrier for benzene formation 5 ± 3 kJ/mol lower. Barriers to diffusion and formation of C 4 H 4 and C 6 H 6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C 2 H 2 islands is included.