Continuous UV-irradiation as well as flash-photolysis of derivatives of 2,6-diphenyl-s-hydrindacene- 1,3,5,7-tetrone (bisindandione - BID), substituted in the p and p' positions of the phenyl rings, was found to result in the formation of stable photoproducts, of which the absorption, emission and excitation maxima coincide with those of the corresponding diarylidene-benzodifurandiones (bis-benzylidenephthalides - BBPh). Studies of light intensity effects gave evidence that this isomerization proceeds via a semi-isomerized intermediate. Additionally, flash-photolysis of BID showed the formation of short lived transients, which, however, are not the precursors of the stable photoproducts. These transients were assigned to 2,6-diphenyl-shydrindacene- 1,3,5,7-tetrone-2-yl free radicals. Their UV-absorption maxima are dependent on the polarity of the solvent and show a bathochromic shift upon substitution.