Laboratory scale experiments were conducted to investigate the effect of a phytoremediation system using poplar trees on the smear zone-soil gases for petroleum hydrocarbons contaminated soil. Eight clear polyvinyl chloride (PVC) columns (1.5 mx0.1 m) were packed with the contaminated soil covered with clean top soil. Five ports were installed on the front side of each column for soil gas sampling. Hybrid poplar trees (Populus deltoides X nigra DN34) were grown in 4 columns. The remaining four columns were used as controls (no trees). Three of the tree columns and 3 of the controls had induced air flow. An experimental apparatus was established to allow for the induced air to flow through the air flow columns. Average air flow rates of 0.43 L/d, 0.71 L/d and 1.50 L/d were tested. The columns were operated for 78 days. During the experiments, the effect of oxygen addition, through the induced air, on the development of poplar roots in the hydrocarbons contaminated soil was investigated. In addition, the compositions and concentrations of the soil gases at different soil depths were evaluated during the course of the experiments.