The reaction between gas-phase acetylene and alumina-supported palladium saturated with C-13-labelled vinylidene is studied using both one-pulse, C-13 magic-angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products to probe the reaction pathway. The presence of vinylidene on alumina-supported palladium is confirmed by comparing the infrared spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR that a high pressure (similar to 350 Torr) of gas-phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene is formed catalytically on the same sample. The resulting benzene incorporates two C-13 atoms. This indicates that benzene is formed by a slow reaction between gas-phase (C-12-labelled) acetylene and adsorbed vinylidene ((CH2)-C-13=C-13=) to form a C-4 intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111), where it was found that benzene synthesis also proceeds via a C-4 intermediate, in this case formed from two adsorbed acetylenes.