Large amounts of chlorofluorocarbons (CFCs) have been released into the atmosphere, resulting in the destruction of the ozone layer in the stratosphere. There is thus a need to develop air conditioners that do not use CFCs. As one such air conditioner, we propose a combined system of dehumidifying cells that use proton exchange membranes (PEMs) with a water evaporation air cooler. PEMs are the solid electrolytes for polymer electrolyte fuel cells and can transfer water molecules with protons. The performance of existing dehumidifying cells using PEMs is not well understood. As important factors to determine the dehumidifying performance, in this study we first measured the transmissibility and the electro-osmotic coefficient n(d) of water molecules through the membrane and electrode assembly (MEA), water vapor diffusivity through the gas diffusion layer (GDL), and the mass-transfer coefficient between the channel flow and the GDL. These measured factors were then adopted in a performance analysis of our dehumidifying cell. Our simulation code solves simultaneously the conservation equations of mass and energy with an equivalent electric circuit of the cell. The calculated results describe well the experimental dehumidifying performance. By extending this simulation code we predicted the coefficient of performance (COP) of our novel air-conditioning system. The calculated COP for our test cells is small at 0.10 or 0.21, but can be made as large as 4 if n(d) of PEM can be increased to 5. (c) 2005 The Electrochemical Society.