The effect of thermal stress on the antioxidant system was investigated in two invasive plants, Eupatorium adenophorum Spreng. and E. odoratum L. The former is sensitive to high temperature, whereas the latter is sensitive to low temperature. Our aim was to explore the relationship between the response of antioxidant enzymes and temperature in the two invasive weeds with different distribution patterns in China. Plants were transferred from glasshouse to growth chambers at a constant 25 degrees C for 1 week to acclimatize to the environment. For the heat treatments, temperature was increased stepwise to 30, 35, 38 and finally to 42 degrees C. For the cold treatments, temperature was decreased stepwise to 20, 15, 10 and finally to 5 degrees C. Plants were kept in the growth chambers for 24 h at each temperature step. In E. adenophorum, the coordinated increase of the activities of antioxidant enzymes was effective in protecting the plant from the accumulation of active oxygen species (AOS) at low temperature, but the activities of catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), and monodehydroascorbate reductase (MDAR) were not accompanied by the increase of superoxide dismutase (SOD) during the heat treatments. As a result, the level of lipid peroxidation in E. adenophorum was higher under heat stress than under cold stress. In E. odoratum, however, the lesser degree of membrane damage, as indicated by low monodehydroascorbate content, and the coordinated increase of the oxygen. Detoxifying enzymes were observed in heat-treated plants, but the antioxidant enzymes were unable to operate in cold stress. This indicates that the plants have a higher capacity for scavenging oxygen radicals in heat stress than in cold stress. The different responses of antioxidant enzymes may be one of the possible mechanisms of the differences in temperature sensitivities of the two plant species.