ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) are one of the contaminants of concern in coastal marsh environments which are subject to crude oil spills. A laboratory scale mesocosm can be used to complement field study of PAH degradation in coastal marshes. Coastal marsh wetland features, such as its soil, tidal cycles, and flushing, that may play roles in PAH degradation can be simulated in a laboratory mesocosm. The laboratory mesocosm tank is made of acrylic as the main construction material with an air chamber inside the tank which functions as a pneumatic system and tidal water storage compartment. Two trays filled with contaminated marsh wetland soil are situated at two different levels: the lower one is constantly submerged while the higher one is intermittently drained. When the air pressure inside the air chamber is high, the water will flow out from the air chamber to the tank to create high tide. When the air pressure inside the air chamber is low, the water will flow back from the tank to the air chamber to create low tide inside the tank. The tidal water sits in the air chamber until the next high air pressure. The cycles of air pressure inside the tank are controlled by an electrical air pump connected to a timer. The experimental setup can consist of several replicates with an air chamber inside each replicate is controlled by a master pneumatic tank. The model PAH contaminant used in the experiment was phenanthrene, a three-benzene-ring PAH, which was spiked to the wetland soil. The experimental results show that the phenanthrene degradation in the intertidal wetland soil is higher than that of in the subtidal wetland soil presumably due to the availability of oxygen in the intertidal wetland soil. The laboratory mesocosm developed in this study can be used as a tool for examining PAH degradation and other non-volatile organic contaminants.