A novel cleavage reaction of thiamin to give 4-amino-2, 5-dimethylpyrimidine (III) and 2-benzoyl-5-(2-hydroxyethyl)-4-methylthiazole (IV) has been discovered. The reaction involves the treatment of thiamin chloride hydrochloride (I) with two mole equivalents of triethylamine and an excess of benzaldehyde in methanol. The finding was further extended to the reactions of thiamin and its homologs with a variety of aromatic aldehydes to give several new 2-acylthiazoles. The pyrimidine moiety of thiamin was indispensable for these reactions. On the other hand, the reaction of thiamin with cinnamaldehyde gave 2, 9a-dimethyl-9-(2-hydroxyethyl)-7-(3-methoxy-3-phenylpropionyl)-5, 9, 9a, -10-tetrahydro-7H-pyrimido [4, 5-d] thiazolo [3, 4-a] pyrimidine (XV).