The crystal structure of an endoperoxide of tetrabenzo[de,hi,op,st]pentacene (TBPA-O-2) was determined by X-ray crystallography. The unusually long O-O and C-O bonds were rationalized by the characteristic molecular structure of TBPA-O-2. The photochromic behavior between TBPA-O-2 and TBPA was discussed based an the enthalpy difference between TBPA-O-2 and TBPA + O-2 and the thermodynamic parameters of the extrusion reaction of oxygen from TBPA-O-2. Copyright (C) 1996 Elsevier Science Ltd