Laboratory and numerical modeling studies were conducted to investigate the effects of solute concentration-dependent surface tension on unsaturated flow and transport of a dissolved organic compound. The laboratory experiments were conducted in a 2 m long unsaturated/saturated sand column with a water table at the bottom and butanol as the solute. Pressure head was measured with tensiometers connected to pressure transducers, and water content was measured using time domain reflectometry probes. The results show that changes in surface tension caused by changes in concentration of a dissolved organic compound can have significant effects on unsaturated flow. A highly localized large flow perturbation was shown to be associated with the solute front. Relatively good agreement was found between the numerical model and experimental results. The results suggest that in many cases where dissolved organic solutes are present in the vadose zone their effects on surface tension should be included in conceptual and numerical models. ¿ 1999 American Geophysical Union