Structural modifications of photosensitizers (changes in protonation, ionic state and aggregation state) under different environmental conditions should be precisely determined to understand the interaction of the photosensitizers with biological systems. In the present study partition coefficients of hematoporphyrin IX (HpIX), disulfonated meso-tetra-phenylporphine, meso-tetra(3-hydroxyphenyl)porphine (mTHPP) and meso-tetra(3-hydroxyphenyl)chlorin in the 1-octanol–phosphate buffer system were determined in the pH region 4.0–8.0. Only the partition coefficients of HpIX and mTHPP were found to be pH dependent. Computer processing of fluorimetric titration data was applied to estimate pK_a values of the imino nitrogens of mTHPP. Monoprotonated species of mTHPP seem to be unstable or nonexistent. The possibility that both imino nitrogens of this dye are protonated according to a common pK_a is proposed. The pK_a value of the imino nitrogens of mTHPP was found to be 2.99 ± 0.04 after the application of a model taking aggregation of the drug into account. The contributions of various aqueous ionic species of mTHPP as functions of pH were calculated and compared with partition coefficients.