Abstract： This paper presents a numeric estimation of MTBE transpiration stream concentration factor (TSCF), an important parameter for the design of engineered MTBE phytoremediation systems, using the experimental data from plant uptake tests in a carefully designed bioreactor at 15～25 ℃. Pre-rooted willow (Salix alba) were kept in an Erlenmeyer flask of 500 mL with approximately 500 mL spiked aqueous solution for 9～12 d. MTBE concentrations were 4.81～6.60 mg/L. Samples were taken directly from the solution with a needle and injected to a purge and trap unit. Results from willow uptake tests indicated that the fraction of MTBE removed from the test systems strongly correlated with volume of water transpired in a temperature range of 15～25 ℃. The relationship between percent of mass reduction in aqueous solution and volume of water transpired was used to determine the observed TSCF value for MTBE. Due to the volatilization of MTBE involved in this test, the corrected TSCF values for MTBE were computed to be 0.58, 0.75 and 0.49 at 15 ℃, 20 ℃, and 25 ℃, respectively. These results indicated that the uptake of MTBE by willow was in a restricted passive manner and MTBE was unable to significantly transport through the transpiration stream of rooted vascular plants. Fig 2, Tab 1, Ref 11