- Abstract Porphyrin binding to serum albumin was studied at the molecular level probing the effects of: porphyrin self‐aggregation, porphyrin species, temperature and protein‐bound fatty acids. Human serum albumin was found to have a single high‐affinity site for porphyrin monomers, with binding constants of 2 x 106, 5 x 107 and 3 x 108 (37o C, neutral pH, M−1), for hemato‐, deutero‐ and protoporphyrins, respectively. Three equilibria models for the dimer binding were developed and tested. The data were found to fit best with a model proposing a single high‐affinity binding site for the dimer, independent of and different than the monomer site. The binding constants of the hematoporphyrin and deuteroporphyrin dimers to human serum albumin (37o C, neutral pH, M−l) being 4 x 10* and 5 x 108 respectively. The temperature dependence (Dp and HSA, 22‐37o C) of the monomer binding showed the process to be entropy‐driven (δGo= ‐45 kJ mol−1; δSo=+146 kJ mol−1; δHo= 0 kJ mol−1). For the dimer binding, the enthalpy change was found to be highly temperature‐dependent implying continuous changes in the heat capacity of the system over the entire temperature range, the trend at the 37o C region fitting an entropy‐driven process. The monomer vs dimer differences in temperature dependence strongly support separate and independent binding sites for these species. Similar thermodynamics were determined for fatty‐acid carrying as well as for fatty‐acid free HSA, with mild quantitative (but not qualitative) shifts.