It is well known that porphyrins and their metal derivatives have attracted significant attention in many areas because of their spectral properties; their characteristic absorption spectrum consists of an intense (Soret or B) band close to 400 nm and a less intense (Q) band in the region 500–700 nm.6,7 They make a core in many other types of molecules, such are chlorophyll and hemoglobin, as well as in many enzymes where they play a heme role.8 In numerous biological and solar energy applications,3,9 it is often important to examine the basic properties of porphyrins and their metal-complexes when they are irradiated by UV light (e.g., UV-B). Treatment of chlorophyll with strong UV and visible light in solutions and in isolated photosynthetic organelles results in irreversible breakdown of chlorophyll, i.e., bleaching.10–12 This work deals with the stability of two porphyrin derivatives, i.e., pheophytin and mesoporphyrin, and their heavy metal complexes to continuous UV-B irradiation. The irradiation was performed in 95 % ethanol for different irradiation periods, providing possibilities for kinetic analysis. EXPERIMENTAL