The photolysis of ethylene has been studied at pressures from 50 to 3000 Torr using a pulsed ArF excimer laser at 193.3 nm. Major products were acetylene,n-butane, 1-butene, ethane, and 1,3-butadiene, with smaller amounts of propane, propene, methane, and allene. Quantum yields varied with pressure and reaction time; the latter dependence is ascribed to secondary photolysis of butene and butadiene. The reaction products are accounted for by three primary processes:followed by reactions of H,and C2H5radicals. The vibrationally excited C2H3radical can decompose to H + C2H2or can be stabilized by collision. The pressure dependence of the quantum yields of the primary processes [1]–[3] is complex, and a photodissociation mechanism involving several intermediates and excited states of ethylene is presented to account for the present results and previous measurements at 185 nm.Keywords:ethylene, uv photolysis.