The importance of pH as a factor relating to porphyrin binding and distribution in different models was examined using for sensitizers, derivatives of chlorin e6 differing in their lateral substitutors. On the basis of experimental data obtained with the use of several different methods (spectral fluorescence, fluorescence quenching, ultrafiltration) we have investigated the influence of pH on pigments affinity to serum proteins (including serum albumin, high- and low-density lipoproteins), porphyrin intramembrane distribution pattern and its mobility across membrane and when membrane-membrane exchange of porphyrin molecules occurs. The affinity of chlorin e6 to serum proteins is very sensitive to the protonation of side carboxylic groups. A fraction of the dye that is bound to serum albumin decreases with decreasing pH. In contrast to serum proteins, there is a significant increase of chlorin e6- binding capacity to the model membrane when pH shifts from 7.0 to 5.5. In acid medium, deeper penetration of chlorin molecules into lipid is observed. As compared with neutral medium, more pigment molecules are localized in inner monolayer bulk. The pH dependence of the rate of chlorin e6 molecules exchange from outer lipid layer of donor vesicles to acceptor vesicles is markedly different from that of transmembrane movement. The rate of the latter, slower process increases greatly in acid medium, whereas the rate of intervesicle exchange decreases.