Diacetylene compound, 1,4-diphenyl-1,3-butadiyne, was photolyzed with 5-fluorouracil as a model reaction of the phototoxic conjugated poly-ynes with DNA or RNA and obtained a [2 + 2] photocycloadduct. The structure of the photoadduct was determined by spectral methods and compared with the [2 + 2] photoadducts of 1,4-diphenyl-1,3-butadiyne with tetramethylethylene and dimethyl fumarate.