A non-isothermal and unsteady wellbore flow model for multispecies mixtures is presented. The model uses a rigorous thermodynamic approach to describe the isothermal flash process for phase transitions, and contains both stability analyses and phase split calculations. For two-phase state flow, the thermodynamic restraint for the pertinent gas and liquid chemical potential of each species is strictly met. This process is fully coupled with the relevant flow equations. Two examples of applications of the model are presented: a practical field trial case study and a theoretical problem. In the first example, the simulations agree reasonably well with measurements from the field case study, demonstrating that the modelling approach can characterise the unsteady dynamic process observed. In the second example, we demonstrate that the model can handle multispecies carbon dioxide flow with complicated phase transition processes. (C) 2014 Elsevier Ltd. All rights reserved.