Lichens often grow in microhabitats where they receive more light than they are capable of using to fx carbon. Unless regulated, this excess energy can end up activating oxygen, thereby forming reactive oxygen species (ROS). These ROS can damage the photosynthetic apparatus and other cellular components, causing photoinhibition and photo-oxidative stress. Tolerance to high light theoretically can be achieved in a variety of ways, but for lichen photobionts, our knowledge of the precise mechanisms involved is rather fragmentary. Here, we show that tolerance to photoinhibition in the cephalolichen Crocodia aurata could be increased by pretreating thalli at a moderate light intensity for 48 h. Increased tolerance was correlated with increased ability to avoid oxidative stress by dissipating the excess energy as heat, here assessed by measuring non-photochemical quenching (NPQ). Increased tolerance only occurred when thalli were pretreated hydrated. The same pretreament did not increase tolerance in the cyanolichen Sticta fuliginosa, which grows in similar habitats to Crocodia, suggesting that cephalo- and cyanobacterial lichens may require diferent pretreatment conditions. Similarly, tolerance was not increased in collections of the chlorolichen Ramalina celastri from exposed habitats, although additional experiments showed that fuorescence parameters in Ramalina can display considerable plasticity. It seems likely that in “sun” populations tolerance is fully expressed and cannot be further increased. However, the ability to harden Crocodia to photoinhibitory stress could provide a foundation for more detailed investigations into the mechanism of photoprotection in lichen photobionts such as the type of NPQ or the role of antioxidant enzymes. Keywords: Photoinhibition · High light stress · Acclimation · Non-photochemical quenching (NPQ) · Electron transfer reaction (ETR).