Data on the intermolecular forces in crystals of 2,4-substituted porphyrin dimethyl esters (DME’s) are reported. Calorimetric measurements show an anomalous behavior at the melting point for two compounds, and vibrational spectroscopy reveals differences in the N-H bonds. Distinctively high T-m values were found for DME’s with formyl at position 2. The experimental data are combined with theoretical evaluations of intermolecular forces in the crystal lattice. These forces are related to the physical and physiological properties of free and protein-bound porphyrins and hemins.