Novel thermally reversible photochromic dihydrobenzothiophenes 1 and 2, and dihydronaphthalenes 3-6 were synthesized. The colored form 1Q returned to the colorless 1R within 150 min, while the decoloration of 2Q was much slower. On the other hand, dihydronaphthalenes returned very quickly. The decoloration rate was controlled by introducing a methoxy group on the phenyl ring.