Two new dithiazolylethenes, 1a 1,2-bis[5′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene} and 2a 1,2-bis[4′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene}, have been synthesized. Their photochromic behavior has been fully investigated in solution as well as in the crystalline phase. They display both fatigue-resistant and thermally irreversible photochromic reactions, with more than