Abstract—Two analogues of α‐terthienyl, namely 2, 5di(2'‐thienyl) selenophene and 2, 5di(2'‐thienyl) furan have been prepared and their phototoxicities toward several microorganisms have been compared. Dose response studies withEscherichia coliindicate that α‐terthienyl is more active than these analogues. α‐Terthienyl was shown to be the most effective of the three compounds in the photoinactivation of yeast alcohol dehydrogenase. Diagnostic tests showed the participation of singlet oxygen in the photosensitization to different extents by these th