A model describing the microscopic isothermal flow inside a fiber bundle completely surrounded by a resin during a liquid molding process is developed. A distinguishing feature of this model is that it takes into account the liquid/vapor phase transition occurring in the gas entrapped inside the tow. In contrast to the existing void formation models which assume that the entrapped gas behaves as an ideal gas, in the present analysis, the condensation and vaporization processes inherent in the liquid/vapor system under high external pressures are simulated by using the Peng-Robinson equation of state. The numerical results show that the phase transition inside the fiber tow has strong effect on both the void dynamics and its size, thus indicating the need to account for this phenomenon in simulation of liquid composite molding processes. (c) 2006 Elsevier Ltd. All rights reserved.