The photodecomposition of gaseous phthalic anhydride (PA) has been studied by ArF laser photolysis. The transient absorption profiles were strongly dependent on the concentration of the ground-state PA, the foreign gas pressure, and the laser fluence. Both biphenylene and triphenylene were detected as the final products. These results clearly indicated that benzyne (1,3-cyclohexadien-5-yne) was formed during the photolysis. The quenching of biphenylene formation by a foreign gas clearly showed that biphenylene was derived from a highly vibrationally excited (hot) intermediate. The hot PA and benzyne were found to be intermediates of the PA decomposition reaction.