In a fuel processor, a hydrocarbon or oxygenate fuel is catalytically converted into a mixture rich in hydrogen which can be fed to a fuel cell to generate electricity. In these fuel processor fuel cell systems (FPFCs), water is recovered from the exhaust gases and recycled back into the system. We present a simple mass balance analysis based on the assumption that the off-gas leaving the system is just saturated (perfect condenser). The model results in simple expressions for the net amount of water produced, and the critical condition for water-balanced operation in FPFCs. The analysis includes the composition of the hydrocarbon or oxygenate fuel, the air-to-fuel inlet ratio, humidity, ambient temperature and pressure, and the temperature and pressure in the condenser. The analysis can be used to quickly assess under what conditions operation is critical and additional measures or alternative water recovery technologies are required. The simple analysis is in agreement with the more extensive mass balance analysis by Ahmed et al. [Water balance in a polymer electrolyte fuel cell system, J. Power Sources 112 (2002) 519-530] and shows the same dependencies of the water balance on the H/C ratio, condenser pressure, ambient temperature, etc. The analysis shows that as long as these parameters remain invariant, the actual amount of water that is used in the system is of no importance from an overall water management perspective. For instance, high steam loads in the reformer, or in the fuel cell (e.g., in case of a polyelectrolyte membrane fuel cell) do not burden the overall water balance in the least. (C) 2004 Elsevier B.V. All rights reserved.