The detailed dynamic modelling of processes involving aqueous electrolyte systems is complicated by a variety of. factors, including the large number of ionic and non-ionic species formed in water, complex thermodynamics, simultaneous phase and reaction equilibria, and the appearance and disappearance of thermodynamic phases depending on operating conditions. This paper presents a general mathematical model of an aqueous electrolytic system involving one vapour, one liquid and multiple solid phases. The model takes account of phase equilibria, liquid-phase ionic reaction equilibria and solid dissociation equilibria. It is shown how phase appearance and disappearance can be modelled in terms of state-transition networks, and how the overall model can be expressed as a mixed set of differential and algebraic equations of index 1. No special constraints, such as electric charge or chemical element balances, are necessary. The dynamic behaviour of an example batch solid system is used to illustrate the modelling approach. (C) 2003 Elsevier Science Ltd: All rights reserved.