The strong promotion by NO of acetylene cyclotrimerisation to benzene on Pd {1 1 1} has been investigated by fast XPS, supported by LEED and Deltaphi data. Islands of NO act to compress co-existing islands of acetylene. The resulting increase in acetylene density dramatically lowers the nominal threshold acetylene coverage required for reaction. In addition, the threshold reaction temperature is reduced from 150 to <100 K, and increased lateral interactions within the organic domains promote formation of tilted benzene. Co-adsorption of NO with benzene itself accelerates the desorption of both tilted and flat-lying benzene at low temperatures. These factors act to increase the yield of reactively formed benzene to 100%, completely suppressing carbon deposition that would otherwise result from decomposition of reactant and product. (C) 2002 Published by Elsevier Science B.V.