Unsaturated flow and solute transport models in isothermal porous media generally assume surface tension at the air-water interface to be constant. Yet many organic compounds, including a number of environmental interest, reduce surface tension in proportion to their aqueous phase concentrations. An isothermal saturated-unsaturated flow and transport model has been developed that incorporates variable surface tension caused by varying solute concentration. The model was based on the pressure head form of the unsaturated flow equation and was solved using finite element methods. The extreme nonlinearities inherent in the problem necessitated spatial discretization on the scale of millimeters and initial temporal discretization on the scale of milliseconds. A comparison of simulations with and without varying surface tension showed that changes in surface tension could have significant effects on pressure head, water content, water flux, and solute flux. Implications relative to multiphase flow models, unsaturated zone contamination, and remedial technologies are discussed.