The first in situ kinetics investigation of benzene formation from acetylene (2.5 L) on Pd(111) is reported. Laser-induced thermal desorption with Fourier transform mass spectrometry is used to monitor the surface molecular composition as a function of temperature and reaction time. The reaction occurs in a stepwise mechanism, the rate-determining step being the reaction of 1 equiv of acetylene with 1 equiv of C4H4 surface intermediate (formed at similar to 110 K) to form benzene. At low temperatures (less than or equal to 210 K), the reaction is self-poisoning by site blocking, and the reaction ceases with excess acetylene remaining. Heating to 260 K, however, drives the reaction to completion. Below 210 K, the reaction is observed to be pseudo zeroth order in acetylene surface concentration, but follows pseudo-first order kinetics overall. The activation barrier for benzene formation is 43.5 +/- 4.5 kJ/mol with a preexponential of 10(9.8+/-0.1) s(-1). Additionally, butadiene is observed, confirming the presence of C-4 species on the surface.