Abstract The thermodynamics of zinc hematoporphyrin (ZnHP) dimerization and ZnHP-membrane binding were studied. The dimerization equilibrium was determined over the temperature range 19–40 °C, using fluorometric techniques. The dimerization constant obtained at 37 °C (neutral pH in phosphate-buffered saline) is 4.6 (±0.6) × 10 4 m −1. The dimerization was found to decrease with temperature over the range 19–36 °C, the data allowing the extraction of the following thermodynamic parameters for the temperature range 19–31 °C: Δ G 0 = −9.3 kcal/mol, Δ H 0 = −7.4 kcal/mol, Δ S 0 = −6.4 eu. For temperatures above 36 °C the dimerization was found to be temperature independent, giving the following parameters: Δ G 0 = −6.6 kcal/mol, Δ H 0 = 0 kcal/mol, Δ S 0 = 21.2 eu. On the basis of the data the case is made for the existence of two types of ZnHP dimers, differing in the location of the fifth Zn 2+ ligand and in the nature of the contribution of the solvent to the dimerization. For the membrane binding, large unilamellar liposomes served to model biological membranes. The binding of ZnHP to the liposomes was found to be similar, quantitatively, to the corresponding metal-free molecule, namely, fitting a case of one type of site and giving a binding constant of 1600 ± 160 m (neutral pH and 37 °C) which is independent of the length of the porphyrin-liposome.