A recent asymptotic model for the operation of a vanadium redox flow battery (VRFB) is extended to include the dissociation of sulphuric acid—a bulk chemical reaction that occurs in the battery’s porous flow-through electrodes, but which is often omitted from VRFB models. Using asymptotic methods and time-dependent two-dimensional numerical simulations, we show that the charge–discharge curve for the model with the dissociation reaction is almost identical to that for the model without, even though the concentrations of the ionic species in the recirculating tanks, although not the state of charge, are considerably different in the two models. The ability of the asymptotic model to extract both the qualitative and quantitative behaviour of the considerably more time-consuming numerical simulations correctly indicates that it should be possible to add further physical phenomena to the model without incurring significant computational expense.