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The synthesis, characterization and X-ray crystallographic structure determination are described for a stable thiazol-2-ylidene 2. This thiazol-2-ylidene is the first example of a stable, crystalline carbene in which the singlet carbene center bears a sulfur substituent. The carbene 2 is the closest stable analog of the important thiamin (vitamin B,) carbene. Although the thiazol-2-ylidene 2 is sufficiently stable to isolate at room temperature in the absence of moisture and oxygen, it will dimerize to form a normal carbene-carbene dimer (2}*. With the isolation and structure determinations of both 2 and ( 2 4 , this system is the first in which a stable carbene and its corresponding dimer have been isolated and characterized. Additionally reported is the "reduced" 2H-thiazoline 2 . H2 in which the former carbene center in 2 has been reduced to a methylene (CH,) group. Thiazol-2-ylidenes with smaller groups on nitrogen (e.g. mesityl or methyl) are too unstable to allow easy isolation. The dimer from 3,4,5-trimethylthiazol-2-ylidene possesses a very unusual geometry in which one of the former carbene centers is planar while the second is pyramidal.