Among the different applications of supercritical fluids, the use of water submerged jets is common in different processes such as the production of nanoparticles or combustion applications. This paper presents a computational methodology for the modelling and simulation of subcritical and supercritical injection of water in a subcritical co-flow. In this work, we focus on numerical simulation of the process, emphasizing mixing modelling. Subcritical to supercritical submerged water jets in a subcritical co-flow are analysed and validated against experimental data with good accuracy (below 5% of absolute average deviation). The main results show that mixing zones are longer at subcritical temperatures than well above the critical point. Besides, due to the low dissipation rate found, the system is poorly micromixed and complete mixing at the molecular scale is never achieved. A methodology based on the evaluation of a macro-mixing length has been used. Using this method, the presence and location of mixing zones are better identified, showing that when pressure is well above the critical point, fluid-dynamic behaviour is more similar to subcritical conditions. Implications of these results are commented and analysed. (c) 2012 Elsevier B.V. All rights reserved.