The singlet oxygen (^1O_2) deactivating rate constants (k_s) of α-tocopherol (α-Toc) and carotenoids such as β-carotene, astaxanthin and canthaxanthin and their inhibiting activities for ^1O_2-dependentlipid peroxidation (IC_50' that is, the concentration required for 50% inhibition of lipid peroxidation) were investigated in liposomes and compared to those in EtOH solution. Liposomes were prepared from dimyristoylphosphatidylcholine to measure the k_s and egg yolk phosphatidylcholine to measure the IC_50 values. ^1O_2 was generated by photoirradiation using three photosensitizers, Rose bengal and methylene blue, which generate ^1O_2 at the membrane surface or in bulk water, and 2-(1-pyrene) dodecanoic acid, which generates ^1O_2 in the membrane hydrophobic inner region. The k_s and IC_50 values were determined in membranes considering the following two points: (1) antioxidants exist in membranes and their active moieties are distributed in different regions of membranes at different concentrations; (2) dynamics of ^1O_2 in membranes (generation site of ^1O_2 depending on the localization of photosensitizer in membranes, and solubility of ^1O_2 which differs in the site of membranes). In conclusion, the following three factors are experimentally confirmed to be important for the consideration of ^1O_2 deactivating activities of α-Toc and carotenoids in membranes:(1) the concentration of antioxidants (EtOH solution < membrane), especially the local concentration of their active moieties in membranes (in the case of OH-group of α-Toc, 0%, 50-60%, and 40-50% at polar zone, hydrogen belt, and hydrophobic core, respectively, of the membranes); (2) the mobility of antioxidants (EtOH solution > membrane), the mobility of membrane phospholipid (liquid crystalline state > gel state), the mobility of antioxidants in membranes (antioxidants located at one half of the bilayer membrane such as α-tocopherol > antioxidants located across the bilayer such as β-carotene; antioxidants interactive with polar head group of lipid in membrane such as astaxanthin < antioxidants nointeractive with polar head group of lipid in membrane such as β-carotene), and the local mobility of their active moieties (in case of OH-group of α-Toc, membrane surface < membrane inner region); (3) the dielectric constant (micropolarity) at the domains in membranes, that is higher in membrane surface than in membrane inner region, where the active moieties of antioxidants deactivate ^1O_2 (in case of α-Toc, reactivity is high in the solution with high dielectric constant).