The reaction between allyl bromide and previously chemisorbed eta (3)-allyl to form 1,5-hexadiene on Cu(100) at cryogenic temperatures has been examined using reflection absorption infrared spectroscopy and temperature-programmed desorption. Above 110 K, the 1,5-hexadiene formation rate decreases with increasing temperature and is controlled by the residence time of dosed allyl bromide, whereas below 100 K, the rate increases with temperature and is controlled by the reaction of weakly adsorbed allyl bromide with chemisorbed eta (3)-allyl. Above 110 K, a precursor state model is used to interpret the data. The activation energy difference, E-d - E-r, between desorption and reaction of allyl bromide with eta (3)-allyl is 12 kJ mol(-1).