The positron when formed as a result of nuclear decay process (β+) can interact with an electron resulting in a positronium formation (Ps). In the first part of this thesis the formation of positronium in benzene solutions containing various halogenated compounds was studied in the presence and absence of C₆F₆ additives. The observed I₂ values, which are indicative of the number of thermalized positronium atoms formed, showed a good correlation with the dissociative electron attachment parameters of the solute species. The formation and the reactions of Ps were found to be very sensitive to structural changes and phase transitions. Therefore, the positron annihilation technique was applied for studying systems such as micelles, liposomes and polymers which have found widespread applications in today's chemical industry. The micelle formation process was investigated in inverted micelles. A distinct cooperative effect of the solution resulting in abrupt changes in the number of thermal ortho-positronium atoms was observed and studied as a function of temperature, counter ion, length and structure of the hydrocarbon chain in the cationic and anionic parts of the surfactant molecules. The results are being discussed in terms of existing models. The thermotropic transition, which is an important parameter for all membranes and liposomes, was studied in sonicated dioctadecyldimethylam-monium chloride versicles (DODAC). The results obtained provide important information on fluidities and allow further assessments of cationic surfactant vesicles as membrane mimetic agents. The third application deals with the study of styrene-butadiene-styrene block copolymers obtained by casting in toluene, carbon tetra-chloride, ethyl acetate and methyl ethyl ketone. The temperature studies show the appearance of two distinct changes in the positron annihilation parameter λ₂ at -70°C and +85°C. They were attributed to the onset of motion of butadiene and styrene respectively. In addition two other abrupt changes in λ₂ were observed at -14°C and +10°C. The change at -14°C was observed in all films whereas the second at +10°C occurs only in films casted in poor solvents (ethyl acetate and methyl ethyl ketone). Their presence is discussed in terms of phase separation and morphological changes. / Ph. D.