In the study, submodels for describing degradation of organic chlorine (measured as AOX) coming from a mill producing bleached sulphate pulp were developed. The submodels coupled to a hydrodynamic model can then be used in assessing the behaviour of AOX in receiving waters. The data used in developing the models was based on mesocosm-scale experiments carried out in two lakes. Waste waters of two mills were used at two dilutions (2% and 10%). Part of the mesocosms were darkened in order to study the effect of light on degradation. In the first phase of the modelling first-order kinetics with a constant coefficient were used. The next steps were to include the effect of temperature and light in the model. Including temperature and light did not much contribute to the modelling results. It was found that more than half of the degradation was caused by the light-dependent component of the degradation reaction. The reaction obeys first order kinetics satisfactorily. However, the reaction coefficient is greater in the beginning of the experiment than later. A correction function describing the decrease of the reaction coefficient was included in the model. The correction function is based on the idea that the most readily decaying components of AOX disappear from the solution in the beginning and therefore the apparent reaction coefficient decreases. Only data on AOX was available in this study but the approach can be applied for different chloro- organic compounds and also for other organic substances.