The intramolecular proton transfer of perylenequinone (PQ) in the ground state S,, and the excited states S, and T, has been studied at the HF/CIS/6-31g level of theory. It is found that the process can be viewed as a proton transfer in S-0 and S-1, while it is coupled with an electronic transfer through the conjugated system in T-1. The barrier of proton transfer in the S-1 is 10.38 kJ/mol. which agrees with the experimental result of hypocrellin A and offers good evidence that the experimental observed process of hypocrellin A is also single proton transfer. (C) 2002 Elsevier Science B.V. All rights reserved.