The chemistry of propyne on Cu{111} is very different from that of ethyne. Instead of trimerising, propyne undergoes coupling reactions in which two molecules react to yield either benzene, with elimination of hydrogen, or C-6 dienes. These two processes occur with approximately equal probability. A plausible mechanism for benzene formation involves head-to-tail interaction of two propyne molecules. 1,4-cyclohexadiene is a likely intermediate in this process, its formation being rate determining. Propyne trimerisation is sterically inhibited by the methyl group. However, the first stage in this process does seem to occur, namely formation of a dimethyl-metallopentacycle intermediate. This may be ascribed to head-to-head coupling of two propyne molecules. This metallocycle undergoes either hydrogenation to C-6 dienes or disproportionation to butadiene and ethyne.