For spark ignition engines, the fuel-air mixture preparation process is known to have a significant influence on engine performance, exhaust emissions and fuel economy. In this work, a one-dimensional, unsteady, multicomponent, multiphase flow model has been developed to study the mixture formation process in the intake manifold for a port-injected gasoline engine. The model consists of three major parts: a gas-phase model, a multicomponent droplet vaporization model and a liquid-film model. Three subsets of equations are solved by a hybrid Eulerian-Lagrangian, explicit-implicit scheme.The model not only quantitatively identifies the effects of each parameter on the final mixture but also shows the interactive influences of three phases of the mixture during the process. As a development and calibration tool, the model helps to understand the behavior of multiphase flow in the intake port, and can give guidelines toward achieving more efficient, clean and smooth engine operation.