An infrared spectrum of a Pd(111) surface collected in the presence of 5 Torr of acetylene as a function of hydrogen pressure reveals that the ethylidyne coverage increases with hydrogen pressure (P(H-2) between zero and 20 Torr). The amount of CO that can be accommodated onto the surface at a pressure of 5 Torr, measured after evacuating the acetylene and hydrogen, increases linearly with hydrogen pressure, and this effect is ascribed to the presence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd(111) and reacts to form benzene, where the benzene desorbs at similar to 280 K. This effect is mirrored in the catalytic chemistry where the rate of benzene formation from acetylene in the presence of hydrogen increases linearly with hydrogen pressure.