Psoralens combined with UVA-irradiation (320–400 nm) are used for the treatment of skin and autoimmune diseases (PUVA-therapy). One possible mechanism of therapeutic and side phototoxic effects of PUVA-therapy can be realized through the stage of formation of psoralen photooxidation products (POP-products). In the present paper, the mechanisms of formation of two types of POP-products are discussed. POP₁-products are predominantly formed in solutions at low psoralen concentration and/or under low fluence rate of UVA-irradiation. These POPi-products possess immunosuppressive activity and are detected by spin trap EPR method. POP₂-products are formed under high fluence rate of UVA-irradiation and/or at high psoralen concentration. They possess hemolytic activity and are detected by Fe(ll)-induced chemiluminescence. The influence of Fe(ll)-ions on POP-induced membranotoxic effects is discussed.