Plant physiological tolerance to oil exposure has been linked to flooding tolerance, as both conditions induce anoxic conditions in the soil. It was hypothesized that tolerance of wetland plants to oil was due to enhanced development of root aerenchyma, which could aerate submerged tissues and support aerobic respiration during anoxic conditions resulting from oil exposure. In this study, four wetland grasses of varying tolerance to anoxic conditions were subjected to 6 L m^-2 motor oil in greenhouse experiments for six weeks and root aerenchyma was assessed by microscopy. Physiological tolerance to spilled oil came from an ability to supply oxygen internally to submerged tissues and to anatomical barrier layers that maintain integrity of root structure. The most oil tolerant species, Phragmites australis (common reed) and Spartina alterniflora (smooth cordgrass), had extensive root aerenchyma, which could ventilate oil-coated roots and support aerobic respiration. Moderate oil tolerance was observed in Spartina pectinata (prairie cordgrass), which was supported by root aerenchyma development, but with minor damage in structure under oil-impacted conditions. The most sensitive species to oil, Phalaris arundinacea (reed canarygrass), had less aerenchyma in roots and extensive root damage from oil exposure. Complete degradation of the root cortex was observed in Phalaris arundinacea and, therefore, a reduction in aerenchyma. These data help explain physiological differences among species exposed to spilled oil. Root aerenchyma provides aeration to support respiration in oiled roots, and an effective barrier at the exodermis helps to maintain root cortical structure.