The photoreactions of hypocrellin B (HB) with thiol compounds (RSH) in ethanol buffer solutions have been investigated by electron spin resonance (ESR), α-phenyl-N-tert-butyl nitrone (PBN) spin trapping and quenching experiments. The mechanism of photothiylation of HB is proposed to involve a nucleophilic anion (RS - ) addition of Michael type at the 5 or/and 8 positions followed by oxidation. The effects of oxygen and pH value have been studied. The photoreaction and thermal reaction have been compared.