N,N'-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI) is an aromatic compound that presents photochemical properties and it can be easily reduced, giving rise to stable imide anion radical able to reduce cytochrome c (cyt c). In this work, we investigated the ability of irradiated PNDI to protect isolated rat liver mitochondria and cyt c/PCCL liposomes against oxidative damage induced by Fe²⁺/citrate and t-BuOOH or HOOH. Lipid peroxidation was estimated by malondialdehyde (MDA) measurement and the changes in the cyt c structure were evaluated by UV-vis spectrophotometry. Irradiated PNDI was able to decrease the extent of the lipid peroxidation with all inducers used. In the cyt c/PCCL system, cyt c alone was sufficient to induce damage on the lipids due to the attack of heme iron on peroxidized lipid acyl chains. In this condition, the presence of irradiated PNDI protected the lipids due to its capacity to reduce heme iron and inhibit the reaction of peroxides with cyt c. Thus, the protection mechanism seems to involve both scavenger of free radicals and heme iron reduction. The addition of t-BuOOH in the cyt c/PCCL system did not change significantly the degree of lipid damage but the addition of HOOH led to a significant decrease in the cyt c-promoted lipid peroxidation, suggesting that the peroxides compete with lipid acyl chain for the reaction with heme iron. In the case of HOOH, the observed protection could be attributed to its partition preferentially out of the lipid bilayer. In the presence of peroxides, irradiated PNDI was also able to protect the lipids. After the characterization of the photoprotection in the liposome model systems, we tested if this property could occur in a biological system. In isolated rat liver mitochondria, irradiated PNDI was also able to inhibit approximately 40% of the Fe²⁺/citrate-induced lipid peroxidation. Further experiments are necessary to elucidate the mechanisms of photoprotection in isolated mitochondria, but these results point to a potential biological utilization of this compound. Supported by FAPESP, CNPq and FAEP-UMC.