Mixed monolayers of dipalmitoylphosphatidic acid and two pyrene-labelled fluorescent analogues of dipalmitoylphosphatidylcholine were studied at the air-water interface, The pyrene moiety was covalently bound to the phospholipids in two different positions : at the end of one of the aliphatic chains and in the polar head group. Monolayers were characterized by measuring surface pressure- and surface potential-area isotherms, and the fluorescence and reflection spectra as a function of compression as well as monolayer composition. Information on the structural and dynamic properties of the monolayer were obtained from excimer-to-monomer intensity ratio, and from the change in the relative intensities of the vibronic peaks in the monomer emission bands. The photoinduced electron transfer between the fluorescent probe and the absorbed electron acceptor (methylviologen) depends on the pyrene location. When pyrene is located in the hydrophobic region of the monolayer, quenching of the excited state of pyrene occurs as a result of long-range photoinduced electron transfer between pyrene and methylviologen. However, when pyrene is located in the interfacial region of the monolayer, the observed decrease in fluorescence intensity is also due to the formation of a ground state complex between pyrene and methylviologen.