Photochromic cyclization reaction efficiency and fluorescence of colloidal solutions, amorphous films, and polymer cast films of two dithienylethenes were studied. Photochromic cyclization reaction efficiency of colloidal solution of cis- 1,2-dicyano- 1,2-dithienylethene (1) was as high as that of hexane solution of1. Amorphous films of1also showed high efficiency. On the other hand, neither colloidal solution nor polycrystal of 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (2) was photochromic; only amorphous films of2were colored very slightly by UV irradiation. While photochromic cyclization reaction efficiency of1depends little on the kind of polymer matrix, that of2depends significantly on the Tg (glass transition temperature) and polarity of polymer matrix. Long-wavelength fluorescence which can be assigned to the colored closed-ring form was observed for1in the solid states, though deeply colored hexane solution did not give any long-wavelength fluorescence.