Mathematical modelling is an essential tool for the design of solid oxide fuel cells (SOFCs). The present paper aims to report on the development of a dynamic anode-supported intermediate temperature direct internal reforming planar solid oxide fuel cell stack model, that allows for both co-flow and counter-flow operation. The developed model consists of mass and energy balances, and an electrochemical model that relates the fuel and air gas composition and temperature to voltage, current density, and other relevant fuel cell variables. The electrochemical performance of the cell is analysed for several temperatures and fuel utilisations, by means of the voltage and power density versus current density curves. The steady-state performance of the cell and the impact of changes in fuel and air inlet temperatures, fuel utilisation, average current density, and flow configuration are studied. For a co-flow SOFC operating on a 10% pre-reformed methane fuel mixture with 75% fuel utilisation, inlet fuel and air temperatures of 1023K, average current density of 0.5Acm−2, and an air ratio of 8.5, an output voltage of 0.66V with a power density of 0.33Wcm−2 and a fuel efficiency of 47%, are predicted. It was found that cathode activation overpotentials represent the major source of voltage loss, followed by anode activation overpotentials and ohmic losses. For the same operating conditions, SOFC operation under counter-flow of the fuel and air gas streams has been shown to lead to steep temperature gradients and uneven current density distributions.