The rate constant for the disproportionation of Cl−2 in water has been determined for the temperature range 22–87 °C using pulse radiolysis of 0.1 M NaCl+1 mM HClO4 aqueous solution. The rate constant for the decay of Cl−2 has been found to be 2×(7.35±0.53)×108 M−1 s−1 at 22 °C (at zero ionic strength). The determined activation energy, Ea=10.89±0.37 kJ mol−1, is less than expected for diffusion-controlled reactions. A good fit to the Noyes equation (1/kobs=1/kdiff+1/kreact) has been obtained assuming the reaction step is activationless with kreact=A′T.