Supercritical Water Oxidation (SCWO) is a powerful process for the treatment of organic wastewater. It has already been studied for decades and, nowadays there are several demonstration and industrial plants, under construction worldwide. Simulation tools play an important role, both to predict and optimize the operating conditions during the process and to evaluate its economic viability. In the case of SCWO, transitory state models are sparse in the literature. In order to exhaustively know the behavior of the SCWO process during time-dependent applications, a transitory tubular reactor model has been built. That model has been developed by the finite difference method in two dimensions. It takes into account the reaction of wastewater along the reactor and the heat loss from the fluid to the atmosphere in a radial way, both along the pipe and through the isolation material. In order to validate the model, a set of experiments under different operating conditions were carried out in a pilot plant. Their aim was to characterize the heat loss in the system to fit both simulated and experimental data in steady state. Then, the transitory model is tested and typically time-dependent applications at industrial scale are analysed under different operating conditions (varying feed concentrations, reactor inlet temperature or amount of cooling water injection). In each case, the model results are collected and they show the time variation according to the changes in operating parameters such as temperature, feed concentration or heat loss along the reactor. The model developed can also be used for other technologies based on supercritical water, such as supercritical water gasification. (C) 2016 Elsevier B.V. All rights reserved.