This review illustrates the distribution properties of some photosensitizers, namely hematoporphyrin (HP), protoporphyrin (PP) and Zn(II)-phthalocyanine (ZnPc) in the membranes of mitochondria and endoplasmic reticulum (ER) from liver of Wistar rats. The dye distribution pattern in the biological membranes is modulated by a number of factors including the dye hydrophobicity and the nature of the carrier which transports the dye to the membrane. A discrimination between protein sites and lipid domains is presented. Formation of porphyrin planar dimers/aggregates endowed with spectroscopic properties comparable to those of the monomeric species is observed in some specific conditions of localization. This phenomenon occurs in the inner mitochondrial membrane but it is absent in the outer membrane and in the endoplasmic reticulum. The possible implications of these findings on the nature of the photosensitization targets are discussed.