In phosphoric acid fuel cell (PAFC) stack, reduction of the phosphoric acid that is impregnated in the cell is a major factor to restrict the operating life. In this paper, the phosphoric acid reduction by the evaporative and condensational dissipation was evaluated by numerical analysis. The calculations that include the behavior of heat transfer, gas flow, and evaporation and condensation of phosphoric acid were conducted for the model cell with conditions that correspond to an on site stack. The phosphoric acid was considered as a composite of phosphorus pentoxide and water, and the evaporation and condensation rates were derived based on the nucleation theory for a two components system. The phosphoric acid distributions in the vapor phase and liquid phase at the process gas and electrode were calculated for the duration of 3300 h, and as a result of those, the exhausting and remaining amount of phosphoric acid for the cell were evaluated. The analysis results were compared wills the experimental results for the model cell. (C) 2011 Elsevier B.V. All rights reserved.