We studied how a plant-derived biosurfactant, saponin, could increase the biodegradation of slowly desorbing polycyclic aromatic hydrocarbons (PAHs) in creosote-contaminated soil. The desorption kinetics of nine indicator 3-, 4- and 5-ring PAHs were determined by Tenax extraction, a necessary tool to optimize the role of saponin by predicting the optimal time for application. The desorption data revealed that initially, 84 % of the total PAHs initially present in the soil (3902 mg kg-1) were fast desorbing. Saponin was applied to the soil after 56 d of solid-phase biostimulation, when the majority of the PAHs that remained in the soil were slow desorbing. The biosurfactant enhanced, in this way, the biodegradation, while the initial addition of biosurfactant had little effect. The stimulation was evident by a 94 % loss of the total PAH initial content in the soil. The kinetics of desorption determined at the end of the bioremediation revealed that direct enhanced desorption, rather than solubilization, was the responsible mechanism, as evidenced by significant increases in the fast desorbing fractions caused by the sorbed biosurfactant. These results show that the appropriate integration of natural surfactants promotes the biodegradation of slow-desorbing fractions and improves bioremediation performance.