Allyl bromide, CH2CHCH2Br, adsorbs both molecularly and dissociatively on Pt(111) at 95 K. The multilayer desorbs at 130 K and the monolayer al 156 K. At exposures >0.63 monolayers (ML), a broad high-temperature parent desorption peak appears, centered at similar to 225 K. For exposures below 0.38 ML, complete decomposition occurs, and only H-2 and HBr desorb when healed, For these exposures, high-resolution electron energy loss spectroscopy (HREELS) indicates complete C-Br bond cleavage and eta(3)-allyl group, C-(a)H2C(a)HC(a)H-2, formation by 185 K, followed by rearrangement to propylidyne, C-(a)CH2CH3, between 300 and 350 K. Higher exposures result in some hydrogenation of allyl groups to desorb a mixture of propane and propylene at similar to 225 K and propylene at 320 K. HREELS suggests that pi-bonded propylene may form from eta(3)-allyl and lead to the 225 K propylene and propane desorption. H-2 and D-2 coadsorption experiments indicate that propane forms only in the presence of surface hydrogen. For these higher exposures, eta(1)-allyl (propenyl, C-(a)H2CHCH2) fragments form and undergo reductive elimination to propylene at 320 K and reorient in a competing reaction to eta(3)-allyl groups that subsequently rearrange to propylidyne between 320 and 350 K.