A dihematoporphyrin ether-free hematoporphyrin derivative has been prepared by a base-catalysed dehydration of hematoporphyrin with sodium hydroxide. The identification was performed by HPLC and mass spectroscopy (FD-MS). The reaction of hematoporphyrin with l M sodium hydroxide for 24 h yields more than 90% of the monomeric porphyrins.