Cold slows down Calvin cycle activity stronger than photosynthetic electron transport, which supports production of reactive oxygen species (ROS). Even under extreme temperature conditions, most ROS are detoxified by the combined action of low-molecular weight antioxidants and antioxidant enzymes. Subsequent regeneration of the low-molecular weight antioxidants by NAD(P)H and thioredoxin/thiol–dependent pathways relaxes the electron pressure in the photosynthetic electron transport chain. In general, the chloroplast antioxidant system protects plants from severe damage of enzymes, metabolites, and cellular structures by both ROS detoxification and antioxidant recycling. Various methods have been developed to quantify ROS and antioxidant levels in photosynthetic tissues. Here, we summarize a series of exceptionally fast and easily applicable methods that show local ROS accumulation and provide information on the overall availability of reducing sugars, mainly ascorbate, and of thiols.