A water injection scroll compressor to supply clean compressed air to an automotive fuel cell system is researched. The water is used as both the lubricant and coolant in the compressor. A thermodynamic model of the water injection scroll compressor considering leakage and heat exchange for use with an automotive fuel cell system was developed using the conservation of energy and mass equations and the equation of state. The results show that the scroll compressor has nearly isothermal compression when injecting water in it. Increasing the compressor rotation speed increases the discharge loss and the volumetric efficiency of the scroll compressor. The difference between the calculated power and the isothermal power increases as the compressor rotation speed rises, which means the efficiency of the compressor decreases. Increasing the flow rate of water injected increases the indicated isothermal efficiency and decreases the discharge temperature. Under the condition studied, the mass flow rate of water has the greatest effect on the discharge temperature. (C) 2004 Elsevier Ltd. All rights reserved.