The possibility that desiccation tolerance in mosses may be dependent on an ability to process species of activated oxygen was investigated using the tolerant sand-dune moss Tortula ruraliformis (Besch.) Grout and the sensitive minerotrophic flush species Dicranella palustris (Dicks.) ex. E. F. Warb (D. squarrosa (Starke) Schp.). Mosses were desiccated in low and high irradiance and responses of these plants compared to those of dark-desiccated and hydrated control plants. Both desiccated and undesiccated plants of T. ruraliformis had a higher superoxide dismutase (SOD) activity than D. palustris, but had similar, or lower activities of the chloroplastic H2O2-processing enzymes peroxidase and ascorbate peroxidase. In T. ruraliformis, desiccation in both light and dark led to a significant increase in SOD activity, but did not consistently stimulate the activities of peroxidase and ascorbate peroxidase. In D. palustris, desiccation in combination with irradiance led to a decrease in peroxidase activity, but had little affect on the activities of other activated oxygen-processing enzymes. Catalase, an extra-chloroplastic enzyme, was up to 7-fold more active in hydrated T. ruraliformis than in D. palustris, but desiccation resulted in significant decreases in the activity of this enzyme in both species. Regardless of irradiance level, there was a depletion of the anti-oxidant ascorbic acid in both species when desiccated. Only in T. ruraliformis was there a synthesis of γ-tocopherol, and maintenance of α-tocopherol and glutathione concentrations when desiccated in the light. Activities of the anti-oxidant recycling enzymes, dehydroascorbate reductase and glutathione reductase, were not significantly increased by desiccation in either moss. The idea is advanced that the role of activated oxygen-processing enzymes involved in the removal of chloroplastic hydrogen peroxide may be less important than that of anti-oxidants in the determination of desiccation tolerance.