AbstractTemperature is one of the most important factors affecting chlorine decay rates in drinking water systems. In this article, temperature effect on chlorine decay rates in raw and treated waters was studied. Results show that temperature affects differently the fast and slow decay phases, the latter being more sensitive to temperature variations, as higher values of the activation energy parameter were obtained. Accordingly, an improvement to the temperature dependent two reactant model (a parallel second order model), in which the activation energies of each decay phase are distinct, is proposed and successfully used for chlorine decay modelling. In waters from transport and distribution systems, however, the fast decay phase is mostly negligible. In such cases, a single phase second‐order model in which the activation energy parameter is given by the slow phase reaction, is likely to describe temperature effect on chlorine decay accurately.