5-Dibenzothienylthiazoles having an ethoxy or methyl groups at the 2-position of the benzothienyl rings showed photochromism both in hexane and in methanol solutions with photocyclization quantum yields as high as 60 %. Upon the addition of acid to the methanol solutions of closed-ring isomers of dibenzothienylthiazoles, the substitution of the ethoxy group with a methoxy group occurred. The generation