Bipolar membrane electrodialysis is used in a three-compartment configuration to regenerate formic acid and sodium hydroxide from sodium formate. A previous study showed that the diffusion of molecular formic acid induces the loss of acid current efficiency. The present study shows the following results: the diffusion of molecular formic acid through the bipolar membrane explains quantitatively the presence of sodium formate in the sodium hydroxide solution. The loss of acid current efficiency is only due to diffusion of molecular acid through both anion-exchange and bipolar membranes. The sodium hydroxide current efficiency is determined by acid diffusion through the bipolar membrane and OH- leakage through the cation-exchange membrane. The flux of acid diffusion through the membranes is proportional to acid concentration and depends on sodium hydroxide concentration for bipolar membrane and on sodium formate concentration for anion-exchange membrane. The flux rates vary with temperature. A model based on mass balance is proposed to describe the bipolar membrane electrodialysis. Some experimental parameters like volume variations are needed. It is seen that a low temperature is favourable to the process. So does an increase of current density. The nature of anion-exchange membrane also affects diffusion. It is found that PC acid 100 membrane is the less permeable to diffusion of formic acid among the 5 tested membranes. (C) 2008 Elsevier B.V. All rights reserved.