Cavity ring-down absorption spectroscopy is employed to probe Br2 molecular elimination in the photodissociation of 1,1-and 1,2-C2H4Br2 isomeric forms at 248 nm.Their photodissociation processes differ markedly from each other, including quantum yield of the Br2fragment, the vibrational population ratios of Br2(v=1)/Br2(v=0), and temperature effect.