The first application of time-resolved fast X-ray photoelectron spectroscopy as a chemically specific, quantitative probe for surface catalysed organic synthesis is described. The trimerisation of ethyne to benzene over a catalytically active Pd(111) surface has been followed in situ by C 1s core-level XPS. Ethyne reacts to form flat-lying (η) benzene above ∼150 K with a low activation barrier of ∼5 ± 1 kJ mol. At 280 K a tilted (η) benzene moeity is formed on the surface, some of which desorbs, the remainder interconverting to flat-lying benzene above 330 K. Unreacted ethyne is not present above 330 K. Flat-lying benzene begins desorbing ∼500 K, in competition with C-H bond cleavage to form a tilted (η or η) phenyl species. The latter may disproportionate yielding gaseous benzene and carbon.